Underwater robots work similarly to robots that run on land, however the wiring and hardware are generally kept in a different location and heavily reinforced with waterproof seals.
A vessel on the sea often refers to a ship or boat on the ocean. By definition a vessel is a craft designed for water transportation (http://wordnetweb.princeton.edu/perl/webwn?s=vessel)
The situation existed when the positronic brain was damaged, and when the robot convinced itself the rule should be broken "for the betterment of the whole race of humanity."