I wasn't plannig on painting, but it might be a good idea. The print looks very tight and I assume it's watertight. But a bit of paint won't hurt, so thanks for the advice. Second print is now in the works (second front part). I'll have to do some design changes on the 2nd and 3rd aft parts, as I now have decided to have the batteries (custom made Li-ion) inside the potons instead of on top.
The plan is to use two underwater thruster motors mounted on each poton, and steer the boat with differential thrust using a Pixhawk and Ardurover software.