If you could plumb steam to the engine you might be able to do this, but
there's likely no way to put reasomable power in an outboard engine with
the boiler requirements.

There have been a number of successful designs that have accomplished this goal. You won't get 150hp out of it, but certainly enough power to move a small skiff at a decent speed- 8hp or so. Traditional boiler designs are too heavy, but the Serpollet monotube works quite well. Check out Wally Mounster and Terry William's designs.