Have you considered Lego stuff?
I recently bought their City Lines cargo boat - I won't bore you with how I made it R/C - and it is fun! It is also unsinkagurgle! I pushed the thing fully under water, waited for the air to all come out from under the deck, then watched as it gently returned to the surface.
A simple prop, shaft, motor and small battery makes it into a great little 'toy' with all the flexibility that the Lego system offers.
Regards,
Geoff