Eh, everything that goes in the ocean requires a lot of engineering to survive. Batteries can be sealed if needed, electric motors can be sealed too. For example thrusters for ROVs are typically electric motors. They just enclose the motor in a housing and run the shaft out to the prop through a rubber seal. A lot of older stuff looks like it wasn't highly engineered because the important engineering was done a long time ago, and now the stuff is old and crusty. But there was a lot of work done to make gasoline engines work well in the water I suspect. We can do the same for electric.