Nu este domeniul meu, desigur, dar m-am gândit la această soluţie, încercând să o fac să sune cât mai literar. Iată ce spune Dicţionarul enciclopedic (1966) despre insula în cauză:

Salamina - insulă stâncoasă în vestul Aticii, celebră prin victoria navală repurtată aici, în timpul războaielor medice, de flota greacă asupra flotei persane (480 î.e.n.).

Salamis - An island of Greece in the Saronic Gulf east of Athens. In an important naval battle off the island's northeast coast the Greeks, led by Themistocles, defeated the Persian fleet in 480 B.C.
http://www.thefreedictionary.com/salamis

My interpretation of this is that "the all-important seas" refers to the seas in general and that the particular seas/routes which the oarsmen may have sailed on are not important here - the emphasis is on the prowess and the fact that they wanted representation to match it. To paraphrase it in other words:

"Athenian oarsmen wanted political representation to match their strength on the seas, and the seas (in general) were vitally important for Athens."

Taking it a stage further, in English this could be rephrased as "to match their indispensable maritime strength". By saying that their maritine/maritime strength was indispensable (to Athens), you're still implying, by extension, that the seas were of vital importance (they couldn't survive without this prowess). This, then, might be translated as something along the lines of "puterea maritimă indispensabilă", giving you an equally concise phrase with ultimately the same meaning as the original.

I'm sure there must be more poetic ways of expressing this interpretation (and perhaps three adjectives, including "lor", are too many - would it still work without that?) but perhaps this will inspire someone to come up with a better!