Plywood hull and superstructure, scaled to take LEGO or Playmobile figures, ports / windows open to allow figures to be placed inside easily. Finished, so far, in matt green, ready for 9 year old grandson to apply the camouflage paint final touches.

Simple lift off cabin

Recycled toy car innards, remote control, battery container and steering server, the motor had a fixed cog that couldn't be adapted so I had to buy a new one! Also invested in shaft and propellor.
I used hard wood to make the tiller arm, server arm and the rudder itself, on a brass rod.

Sea ( pond ) trials proved it well powered, a bit slow on steering response, but tough enough to take a knock or two. The final coat is an exterior primer with a matt finish.
I will also give him a few "taster" paint pots of suitable "camouflage colours" for him to apply the final finish.
I was thinking of gluing some LEGO base plates onto the decks, so that his figures will be secured when they go to sea.