Decided to read this thread after looking at your latest vids.
I have to say I alsways enjoyed running my waterprrof scale trucks in water. They were low speed setups, with brushed motors. I often used the old tamiya bushings instead of bearings, and packed everything with waterproof grease. I sealed the wheels and tires, or punched holes in the tread, to let the water out. Prefered sealed tires, but this was with the small 1.9 size.
Nifty trick I found, drill an 1/8 inch hole in the back of the motor, so it points downward. This would let the water out pretty quick and helped to get the debris out. I lubed the motor bushings with atf, and would rinse the truck with clear water and use compressed air to clean off the motor, etc.
I have a blue xl5 laying around, and a few brushed motors. And a selection of crawler parts. Maybe time to make another "deep water" vehicle!! Won't be quite as cheap as you did, but I have all the parts laying around, so I will not spend any money now!!