I have a closed cooling system in my prop boat (don't tell anybody I own one), and I like it. Pros are: consistent running temperatures, no winterizing (at least here in CA, some closed systems need to have the exchanger drained of raw water to avoid freezing in cold climates), less salt/sand/silt/debris in the motor. Cons are: need a second water pump of some type either to circulate raw water or coolant, weight will be higher from the added components, increased cost, and the system will be a little more complicated and add a few more things that can go wrong in a day of boating (that's not to say that a closed system is not reliable, just that if you add parts, those parts can fail).
I see it on a lot of the bigger, non race aluminum boats. It's a nice system and if you are boating where you need it, then by all means go for it. It will look a little out of place on a low freeboard California style jetboat, but whose to say you can't make some cool billet brackets, add some braided lines and chrome the exchanger! There are quite a few small things that can be done to a standard motor/cooling package to increase its ability to live in a hostile environment, ie coatings, paint, flushing, routine maintenance etc. I know its not very specific, but I hope this helps a little-Aluminum Squirt