These are probably brush-less motors, so in theory, they shouldn't pose any specific challenges in that scenario, as there are no exposed conductors, only sets of insulated coils/windings and permanent magnets that would end up being immersed in a river.
EMP sensitive?
Not the motors. They are designed to withstand high currents.
>How easy is it to insulate the electric motors?
Very easy. There are submerged electric motors for marine propulsion after all.
True. But they don't take hits and vibrations like a military 4x4 would.