As the Greeks your trade is really important - so Naval dominance in the Aegean is equally so. Blockading the Roman ports nicely keeps them from doing anything out of the ordinary in the Adriatic or invade your provinces. I find the Scipii is the only Roman faction that showed much interest in invading by sea - which was pretty scary for a little while. At this point in the game I don't think anyone matches my naval supremacy. I don't stray too far out of the Aegean and any ships that come near to there or the Adriatic will be leaving sorely defeated from my 3 Gold Chevron bearing fleets.

However, I don't think that ships are that important to other factions that aren't really in need of a navy to unite their provinces.