hmmm i have some 40mm thick balse think I win.
there will be build rules.
I was thinking the same about rules, and thick balsa.
Perhaps one needs to think about a practical design spec, then build one to see if the engineering works out. If it was all balsa there is a possibility that a strong motor would just rip a chunk out of the floor.
Possible solution, but it would need to be tested. If the bow, stern, and bottom are made of the usually ply, it should be strong enough not to collapse under its own weight. If the sides of the hull are balsa, then that would be the point to attack, and defend in your own boat. Perhaps specify 2mm ply. I also seem to remember that balsa usually comes in 50mm wide boards. That itself poses construction limitations.
In addition to no projectiles, or explosives, I would also consider no fires. Wicksteed management may have an objection to that on environmental grounds (polluting the water), and safety (combustibles in an area open to the public). If the latter is raised as a safety concern it could affect the future use of steam. I know it sounds dumb, but we have all seen incidences of the like in the hobby.