K&B makes the SS OB with an air-cooled head and some have taken the Andy Brown MAC's and have used his buggy head on the OB's also. Most .21's don't need a lot of water cooling so air cooling has proven to be an option but for those others that talk about using plane engines in their boats, there simply isn't enough in the way of heat sinks to keep them cool. I threw that in there for them.
I've got 3 .12 monos and 2 .12 'riggers that use car and truck engines. All get plenty of air, a lot more than cars and trucks do under a Lexan body so you can cut a few fins off the top to keep from overcooling the engine.
Cracker Box pretty well hit things right on the head as far as using the engines in boats. The SG shafts are a pain in the rear as even when you cut off the pilot shaft it might not leave enough threads to put the flex cable collet on after the flywheel is installed. Slide carbs can be used but rotary carbs are a lot easier to set up.
A Speedmaster 21 hull would be one of the ideal ones as they do have the air channel to help get the air to the heat sink but any low profile hull should work.

Yeah, an OS .18 in a 24" C-box is a little overkill.