When I lived in Nebraksa and would ride in the winter time, we just always started our ski's for a bit before we put them in the water. Then once in the water just putt around slowly brapping for a minute or 2 and let your engine temps get adjusted. We never had any problems doing that.
If you just put in the water and start ripping, your going to get a cold seizure(piston expanding faster than the cylinder). We never did any kind of restricting, but you sure can.
Good luck and keep warm!