For a closed system it really doesn't matter, except for the fresh water side. The closed system should already have anti-freeze in it. You don't need a block heater until 0F. I doubt you would be out with water below 50F. For the fresh water side, an in-line heater with a pump would be the only way to go. A raw water system would need special plumbing to keep water out of the exhaust when not running.
If it is only a few degrees that you need, a 100w heater (light bulb) in the engine bay would work fine. I have done it many times trying to get a few more weeks on the water with out winterizing. We used one of these in a small pump enclosure. It was good to about -20F with no worries. Now the new boat I have has 30 ft of heater hose and an exchanger that is not easily protected. My only safe option was to put the boat in my garage. It literally fit with 3 inches to spare! Last weekend was 84F. Yesterday and today we got about 8 inches of snow with 50 mph winds! The ice on the hydro lines was about thick as a baseball bat in some areas.