I have an old Sterling models Lumba-Lumba boat, a little bigger than the Tuna boat you have. It has a single Dumas motor in it with a 6 volt 4.5 amp gel cell in it. I can run that boat all day on one charge. For 3 batteries it's hard to say just how long it will run, I would say that it could outlast you. Keep you batteries mounted as low as possible even if this means cutting out the floor in the bottom of the hull. With you having to run your boat in the ocean because of no ponds available this would be highly recommended. The lower the center of gravity the more stable your boat will be.

As for laser cut, 40" w/l sailing vessel that is r/c-able kit, that may be a tough one to find. Here are some more places to try.

http://www.modelexpo-online.com
http://www.acrossthepond.net
http://www.bluejacketinc.com

Hey, ask your dentist for the carbide burrs that he can't use anymore. Those are the ones that fit into the high speed hand piece (drill). After a while they will dull up and won't cut thru tooth enamel but they sure will cut thru a piece of wood. You will need a 1/16" collet to fit them into your Dremel tool. Also broken or dull scalers (picks) that the hygenists use come in handy.