If you go with the paddle-wheeler pontoon system, I'd put a wheel on the inside of each pontoon, so that it rests on the road wheels (on top of the track is probably best), which is linked via a gear & shaft to a paddle-wheel on the outside of the pontoon. The gears are to ensure that the Sea Tank moves as expected by the driver (e.g: forwards in a forward gear or backwards in reverse gear) & allows the rotation of the tracks to assist the motion of the Sea Tank as dictated by the paddle-wheels. By The idea behind ... having the connecting wheel on top of the track (&, preferably, the geared shaft above that) is to raise the paddle-wheel to where its axle/shaft is above the water sufficiently for the paddles to provide forward motion without excessive lifting force being applied to the front & downward force applied to the rear of the tank.
One problem with your idea is that the Crusader tank didn't have (to the best of my knowledge) the capacity for "neutral steering" (turning on the spot with the tracks turning in opposite directions) which limits its manoeuvrability somewhat but shouldn't be a game-killer by any means.
