On the ambient_generic, there should be a field that says something like "entity sound source" or "entity source" (I'm going off memory). If you put in the name of your boat there, the sound should always sound like it's coming from the boat.
Is all of the playable area just within or on top of the boat? If so, you can save yourself a lot of trouble if you just make the boat static, and create an illusion of movement with a scrolling texture (see also: ctf_convoy)
Originally Posted by Bermuda Cake
maybe we should have an Asylum night on the server sometime.