they are designed for use in saltwater, the recommendation is to rinse them well afterwards with fresh water. there are lots of cheaper versions of these ROV thrusters available, I went top of the line units from Blue Robotics, they sell all sorts of replacement parts for them so if there's an issue, it isn't just 'buy a new one'.
you can imagine how fun it was to have the Mirage drive in reverse, slow pedaling to hold against the tide that was pushing me into the submerged rocks, and being able to just swing the rear of the kayak left or right by turning the rudder full to one side and hitting the thrusters for a second or two...all while tapping the bottom with a cannoball sinker and chanting "ling ling, come out and play".