I've probably said this many times here, but it bears repeating:
Those things are designed to exhaust directly down from the chassis of a truck.
In a
boat, you not only have to worry about directing the exhaust outside, but the exhaust hose can get red-hot. Literally. Not something you want anywhere near any flammable
wood or
fiberglass. And since they're not made to use a long exhaust hose, the backpressure may cause it to overheat and shut down. You also have to worry about
water getting into the hose and pooling in any low spot, or, worse, getting back into the heater.
Solve the exhaust problem, and they're great.