If Fire represents Cannons then what does Shock represent for sea warfare? Boarding Actions?
I believe that in EU2, the shock value for admirals was actually called "Boarding" at one point, so your guess is likely correct.
Which would make sense, since land shock consists of hand-to-hand combat. It seems reasonable that naval shock should consist of the same. (The Greeks and Romans did use rams as "shock" tactics, but the practice had become mostly unknown by medieval times.)
Galleys do have far better shock than big ships do at low tech levels. This can be attributed to the fact that galleys, at least on relatively calm seas like the Mediterranean, were far more maneuverable than sailing ships. This advantage disappears as sailing ships become more advanced.