The exhaust from an internal combustion engine is corrosive. Possibly from the water. Stainless steel parts are indicated.
If you fill the entire wake with exhaust, there are health considerations. The cabin might fill with carbon monoxide.
That said I've considered a system that feeds a Coanda nozzle as the end-cap of a boat tail. Reduce the wake to a single vertical line and then inject hot gas at the very end. What could possibly go wrong.
The one at lower left.