Well my thoughts are thus:
That engine delivers high torque figures, so would likely cope with the heavy resistance of water.
It's a reliable 4 stroke engine
It could be connected to a boat gearbox simply due to its shape.
It would be significantly cheaper than an outboard of equivical power, even with additional purchasing of cooling system/gearbox etc
However:
In a car, the engine does not have to run at full power for lengthy periods of time, but in a boat it does.
No I do not have a spare (or even a primary) one to experiment with.
I can't see why this wouldn't work, but it's not done.. So either there's a good reason for that, or people just haven't thought of it yet.