I think they should have some place to rearm, naturally by the docks. It could probably be accomplished by having a ammo crate either on the jetty or underwater.
I think you could also rearm at the litte docks introduced in Invasion of the Philipnes.
Maybe, I just noticed it on Makin (I think..)
if you don't have a harbor i think the destroyer should rearm you. or some other boat.
And what about the destroyer on Iwo Jima? Once it's rockets are gone there is no real use for it except to beach it and use the 20mm guns against the inf. Another thing I have noticed is that sometimes the Torps just go right under the enemy ship and not detonate.