The boat "USE" is freshwater, not the design use of the motor.
It would be air cooled if it does not have a water intake/exhaust. The newer B&S motors are better than the old ones, most now have oil pumps, vs "splash" method of circulating oil. Also quiter than years past. At least assuming lawn mower motors. B&S have lost a lot of the mower market over the yrs with other makes coming in with better engines in the mower field, and they were a bit slow in converting. I do not think I have owned a briggs motor for the last 20 of 30 yrs being in the landscape maintenance field.
I would consider it, not sure how long they have made boat motors, but what they hey, if it works, price is right............what is there to lose, other than it not being what it was advertised to be.
She drives me boat,
I drives me dinghy!