I have a customer who fitted the speaker onto the bulkhead which separates the Y-boat bay from the rest of the hull. Bore a series of holes in the bulkhead or fit some mesh to allow the air from the cone to escape into the Y-boat bay and thence to atmosphere, and be sure to fix the speaker firmly to the bulkhead. Another option is to mount the speaker right in the bow, pointing backwards into the hull, but this would likely compound your model's weight problem.
Whatever you do, make sure that there is sufficient ventillation to allow the air from the front of the cone to escape from the model, and box-in the rear of the speaker to prevent any air from the back of the cone mixing with that from the front (or they will cancel each other out).
You don't need a large battery for the amplifier; a 12v 'AA' size NIMH pack or a 3S LiPo will be fine. At full volume the amplifier is only drawing about 1 Amp.

Dave M

Yes. Look for "mylar speaker". Available in various sizes.

Alternatively, a paper speaker can be put in a plastic bag with a little bag of dessicant. The sound will pass straight through the plastic buty the water wont. Moisture is another story, hence the pack of silica gel. These can be dried out in an oven when they get tired and re-used.