Hi, Jay,
The E-motors in a boat were DC powered from the batteries. They would have been pretty much silent in operation, like the motor that powers an elevator. There may have been some noise from the bearings and brushes running over the commutators, but it wouldn't have been anything like the hum from a large AC-powered motor.
Most of the noise would be coming from the propellors, esp. if they were thrashing fast enough to start cavitating.