From penteconter to trireme

wiki commons, 5th century relief of an Athenian trireme with unprotected rowers

wind independent propulsion - paddles and oars
In an environment of sailing ships, paddles and oars offered the opportunity to translate numbers of humans on a watercraft into motive power. This motive power allows for approaches that can not be made under sail and was a usual procedure for maneuvering into harbour for all kinds of ships. The approach  vectors allowed for beaching to trade or raid on many beaches of the Mediterranean with ships, the penteconter for example, that had not yet split into war and merchant ships. The threat of raids from the sea explains the location of a number of Greek poleis away from the sea. Athens is one such example that needed several kilometers/miles of long walls to connect their city with one of their harbours (there were several more harbours available). A ship that carries more men than goods has usually less peaceful intentions and is more likely to catch under oars a ship with more goods and less men. Piracy in the Mediterranean used suitable known ambush positions that offered favourable conditions of wind and visibility for approaching and boarding.

armed men on the ships - marines

Boarding was the essence of naval power expression that allowed to seize ship with goods and surviving crews. The ability to successfully board was dependent on the training and numbers of marines in comparison to defenders in close quarters combat.

Missiles were not yet capable to destroy other ships, but thin the ranks of defenders and rowers of naval vessels. Bows, javelins and early crossbows (gastraphetes and oxybeles) or ballistae had different ranges, penetration power and rate of shot. Positioning for shots was possible and does make a major difference in missile warfare, because human capability to defend against all these visible (slow and deflectable in comparison to modern gunpowder weapons) missiles was much better than against surprise missiles from unseen angles.

Boarding marines had an advantage from supporting missile platforms and boarding could be conducted from several angles by teaming up on ships, all kinds of flanking maneuvers. Add to this temporary or permanent mobility gains made via breaking oars and masts or ripping sails, with the skillful use of hooks and claws or flooded hulls after ramming damage and you have a way of fighting with increasing inequality. Weaponizing the ship gave one side more options at putting the other at a disadvantage.

wiki commons, Assyrian representation of a direme warship in 700 BC with an elevated platform for marines

a ram suitable for damaging ships
The damage did not make ships sink to the bottom of the sea, but foremost increased displacement to the level of immobility. Ships were full with humans, tools and wood, who could provide a number of stop gap measures against the kind of damage incurred by a naval ram. Impressive as it may look, it could not punch a hole, but only break materials with resulting cracks for water to enter. planks were quite thick and joined by mortise and tenon.

wiki commons, Atlit ram found on the Levantine coast, radiocarbon dated to 530-270BC, the construction of the ram is for maximized fracture of the mortise and tenon timber joints and minimized friction during extraction.

Ideas for speedier standard design production did exist, with ships built around design templates for the interior in the usual way with outer hull first. After finishing the outer construction, the templates were removed and interior could be fitted to the result. The weight of a heavily manned ramming vessel with low dead weight and sleek lines for speed posed a major stability problem due to hogging. That was solved with cables running from stern to bow. Like in modern ships, structural failures could lead to a ship breaking apart under its own weight. As for the naval ram, with an understanding of ship construction and handling critical damage could be delivered that compromised stability to the degree of collapse of the hull, but not with one limited blow. Sea battles between such fleets usually take note of the degree of mutually delivered damage to ships without sinking each other.
Not only hulls were damaged, but oars, rudders and sails as well. The oars could be broken by the sheer momentum of passing hulls, while rudders were a prime targets for grappling with small maneuverable vessels usually employed in defensive encounters. Sails time and again fall prey to missiles that create holes in them.
Pirates used to chase sailing ships with their oared ships having a limited sprint time under oars to close within the range of sight of a sailing transport vessel. Longest ranged missiles suitable against the sail propulsion were of great benefit for the chances of success at capturing these sailing vessels and their content. In numbers and ship height the merchantmen were not yet capable of withstanding such attacks. The large and sophisticated merchantmen of later ages, who could beat off the timeless pirate threat, were not capable of beaching everywhere and could thus only use few suitable harbours with a network of smaller vessels always providing the regional connections from these centers. As of this age, the merchantmen were severely undermanned and had little hope to prevail in close combat.

development history
The development was from the penteconter, as a multi tool for voyages that were trade and raid in foreign lands, beyond own legal regulations with a corresponding multi-purpose crew of warrior, rower and seaman as the job description for every participant. The penteconter remained a trade vessel, but was eclipsed both in war and trade by different designs. For trade more specialized round ships developed that approached the mathematical ideal of a sphere that had most content in comparison to least surface. Hydrodynamics were not optimal for speed that was about five knots with and two knots against the wind, using square rigged sails. In warfare the penteconter received more rowers and a second deck level that elevated the warriors. You could not board a ship with a towering deck full of warriors from below. This gave the new type of penteconter a decisive advantage that enabled them to focus on rowing and damaging other ships. The top level rowers were still warriors. Against a foe in low decked ships they could rather use their superior rowing power to create damage to his ships and use the results in timing to pick the fights of their attacking marines. Picking fights leads to increased success, while a higher position enables fewer men to fend off more.
The milestone in this development of successful ship to ship engagements was the trireme that could smash into the hulls of enemy ships on a degree not possible with the older biremes. For this purpose another level of rowers was added, either slightly elevated from the second level, as in Phoenician designs, or next to the second level with an outrigger, as in Greek designs. Both increasingly aimed at damage of ships and fought more defensive in boarding action with Athens taking the lead in this direction by a massive build-up of very light rowing ships with a small select force of hoplites (The strategos Cimon probably temporarily reverted that Athenian trend). Other navies were more convinced of strong hulls with more marines that maintained ramming as a tactic, but boarding as the decisive element that could be conducted with an enemy not as crippled by ship damage as Athenian marines needed for such a success. Ramming could be used to inflict damage beyond the structural limits of ships that would make them break and submerge as floating wood.

humans on board
Fighting for one or the other advantage was tiring for rowers and marines, while chasing another ship or fleet  tired the pursuer a lot. The pursuer needed a speed advantage that required an more than square energy input increase. Rowing all night to the naval Battle of Salamis in order to catch the allied Greek fleet in the Persian War was criticized as a folly by the Phoenician nobles (who lost their heads for insubordination after the battle was lost). These tolls on each crew made warfare of rowed ships one of the few examples of naval warfare suitable for the use of fresh reserves that could turn the tables.
Manning as well didn't require exact fulfillment of certain figures and likely was understrength with some overmanning by compensating rowers. These rowers increasingly served in a non-close combat role, but should not discounted as light infantry with shields, javelins, clubs and knives. As for close quarters against heavily armed marines they were no match, allowing to board and conquer enemy ships, although on land they could wreak havoc on unexpected and unarmed people. Warfare with these oared ships should be seen in an economic context of enabling to extract valuables from the coasts, where most people settle and wealth is accumulated due to transport. Like each and every robbery, the get-away vehicle was a problem and a highly capable warship that only required a few heavily armed marines nicely solved this issue. As for Athens, the increasing reliance on ship on ship warfare through maneuver reduced the risk to the lives of the people involved in this kind of combat, making naval&amphibious warfare a low risk high gain affair. Such a constellation likely induces to follow down that route of profits with ever increasing naval armament and emphasis on shipborne capability that paid off in the raids on land.
Not only Athens followed that route in combination with island conquests and cleruchies, but Carthage as well, although the Punic had less of these tiny islands and rather more control over low density populated lands except for their North African homeland. From a very similar population and economic base they split their conquest into land and naval at the same time with reduced risk to their population base in the naval&amphibious component. That does not mean that these were outstanding rowers, but that were enticed and served them for easy gains that did not require as much previous capital to invest than conquests on land. After they made their voyage, a safe investment on land might have been envisioned that offered less profit, but lower chances of misery due to weather misfortunes.