When is the last time you saw they fight on ship ? I can only remember during Sanji time.

Where is the ship to ship dog-fight? It seems to me the ship is just transportation in this manga series.

Is it because the power of every single individual can wreck a ship in single swipe that ship is just not a tool for war anymore?

Flying Fish Rider fight.

there were actually a few times though they just didn;t board the ship

The defence against Garp at the end of Water seven

Davy back fight: Luffy Vs Foxy

small fight Luffy Vs that bako factory guy

the escape at enies loby

and you have the war at ace's excucution where there are large parts where they fight from there boats

[img]http://i718.photobucket.com/albums/ww187/D_dragon/omfgpudding-1.jpg[/img]

One of the point is, the Straw Hats don't let any enemies get close to their ship if possible.
If they can hold them off from far away then that's what they're gonna do.
Plus, they are 'kind' pirates. They don't just go and attack someone else's ship. They would rather avoid unnecessary fights (well, except for some hot-tempered you-know-who).