kotori, the model actually does have 1/16'' thick balsa at the waterline:

Edit: The objective of combat in this period was generally not to sink the enemy, but to kill or demoralize the crew into surrender or disable the ship, and take the ship as a prize. That is why I am considering sheeting upper areas with balsa as well to show damage above the waterline. Of course, you could still try to sink the ship, but the multiple bulkheads and bilge-pumps I am going to put in would make it pretty tough. You could, however, shoot small sailor figurines off the deck to disable sails or cannons, or fire at the magazine to set off a smoke-bomb, etc. If I made the rudder out of balsa it could be shot off to disable the ship.

I considered installing a propeller so I wouldn't get stranded but I think it would ruin the feeling of a sailing ship for me. So I have to think of some other way of obtaining auxilary power, one idea I think could work consists of multiple oars attached to belt-driven wheels that could be extended through small ports in the sides of the ship. I know galleons of the 1600's always carried large oars in case of emergency and for maneuvering in port. This would be slightly complex, though, and I doubt I could mount CO2 cannons at the same time, as it would take up a lot of space below the gunports. Maybe just 2 or 4 oars at the back would do it. Has anybody ever built an RC ship that uses oars for propulsion, like an old Greek bireme?

I'm going to concentrate on getting the model sailing properly before I use it in battle. Therefore I am planning to only mount the pyrotechnic display cannons at first. These could technically be used to fire ball bearings but it would be illegal to do so in Canada where I live because they would become firearms then, and not just fireworks. The CO2 cannons would take a long time to devise and implement so the ship probably won't be ready for "safe" combat for a while.


Edit: here's pictures of the plans I used.