I have lots of experience with engines freezing and causing damage over the last 25 years. Oddly enough, I don't see damage done until it gets below 20 degrees for an extended period, meaning at least a day and not just at night, but if it gets down to single digits, it doesn't take long and does more damage. This whole hard freezing thing is an interesting topic since water is actually most dense just above freezing so as the colder ice gets the less dense it becomes. The harder the freeze, the more separation of ice molecules occurs which introduces air into the system which creates more volume, hence overall expansion. On the 4tecs, the only thing really at risk is the intercooler. On the 2 strokes, the 800 and 951 are more at risk due to the angle they sit at for holding water in the cylinders. Although I have seen plenty of the straight up standing motors like the 720 with cracked outer cylinders, just more of the angled motors.
The biggest reason on 4tecs to winterize is keeping the valves and cylinders from rusting which will shorten engine life. I'd say for the op, you don't need to winterize but you should at least start the ski every 3 or 4 weeks and let it run on the hose for 3-4 minutes and you'll be fine.
