Fluid

Kmot: It looks like there may room for a servo in the open well under the motor mount. While mounting a servo in the open may seem like an invitation to disaster, it can be done with good reliability. Use a standard size $10 servo laying on its side, glued in with silicone glue. Waterproof the servo first by coating all seams and screws and wire exits with silicone. Pack the exit shaft under the servo arm with vaseline. This will keep the servo operating fine when wet - just seal the control wires well where they pass into the sealed radio box.

Melting the bottom of the hull with hot batteries is only a risk when the bottom of the boat is out of the water. Melting the hatch cover is more of an issue, but some insulation under the hatch can help. The "fuzzy" part of Velcro can be stuck under the hatch above the cells to give some insulation.

I agree that a gear drive will be needed for this boat if decent performance and run time is desired. You can spin a three-bladed prop with more pitch and gain a low of thrust while reducing the load on the motor and battery temperature. You will also be able to run a more powerful motor for less resistance and possibly longer run times. While the shroud is a great safety idea, it will cause a lot of drag and is not of the correct design to improve thrust.