If you keep your boat inside at night I wouldn't worry about it at all. It would have to drop well below freezing, and stay there for awhile in order to freeze solid enough to crack your block.
I've heard of people putting a drop light in the engine compartment at night to keep it from freezing. A light puts off a lot of heat. I never tried it because I was afraid of it catching fire.
If your really worried about it, get a few gallons of the pink r/v antifreeze(biodegradable/won't hurt the water). After you get done riding, unhook the hose from the bottom of the boat that sucks in the water. Put it in a bucket with the antifreeze. Start your engine, and watch for the antifreeze to come out of your exaust. When you see a good amount coming out, and before you suck the last of it out of the bucket, shut off your engine.
Now your engine is winterized. You don't have to pull plugs.
Don't worry, the antifreeze is more then enough to get you through the coldest winters.