Charles. Having had a quick surf this seems to be a "hot"550 motor.
I would try this with a 35mm prop first and see how it goes. ideally you need to hook up an ammeter or watt-meter to this and see how much current it is drawing before letting loose on the pond.
Are you going to use plastic 2 blade type props or 3 blade brass ones? cant find official specs after some surfing but one test had the motor maxing at 13000 rpm (no load), so plastic may be best to start with,. an "S" pitch 35mm.
Ashley