Not to burst Leviathan1's bubble, but the "as long as water has room to expand when froozen, it will not burst anything, can be a misleading statement.
Yes if you put water in a bucket, and let it freeze, it should not burst the bucket, our water systems are a lot different. it is possible for water to freeze in one part of the boat faster then in another, and cuase the slower freezing water to burst whatever. I would not rely solely on blowing the water out, unless you are 100% sure you got all of the water out.
Run some antifreeze, vodka, gin whatever works for you, you do not need to fill the system if you use the air method, but just back yourself up. Me I wish I could afford to use the cheap vodka, but it is not in the budget. Besides I can just see myself working on the boat in Feb, turning the water pump on for a glass of that magic water.
Yes the water heater should be drained. Install a water heater bypass kit, so you donot have to use a ton of antifreeze.