I've been a regular saltwater skier for over 20 years. My current boat has been on salt for 18 years, and it runs/looks brand new. Here are my thoughts on how to be successful in this environment:
1) need to keep your bilge almost 100% dry. Any moisture under the engine box will start corroding the engine. Check the shaft seal, rudder tube, exhaust system for any leaks. If you do get water in there - which you will, then rinse it out and dry it before putting away. This seems hard to with the boat sitting in the water, much easier if you are trailering your boat in and out of salt.
2) I'm a big believer in closed cooling. I have had it on my boat from day 1. It was a $1000 kit. I've never had any problems with it and last summer when I took my cylinder heads off, the water-jackets looked perfect. Well worth the $1000!
3) Manifolds and risers do turn into a "consumable". You will have to replace every 4 years. The good news is they are readily available (at least for the Ford PCM) and can be changed in a few hours if you are handy. Costs are ~$800 per set, including gaskets.
4) Monitor the riser/manifold interface for leaks. If you see any evidence of a leak (salt trail), fix it ASAP. I always run a file across both interfaces to make sure they are perfectly flat. Also, your a torque wrench to install the riser. I use 25 ft/lbs.
5) My first trailer was painted, it lasted 3 years and the fenders were dangling. I'm on year 17 with my galvanized trailer, it's not a thing of beauty, but it's structurally sound. There are a lot of good options for trailer brakes: Stainless calipers, Galv-X rotors, Stainless backing plates, etc.. These parts last many years (~8 years maybe), and they aren't that much more expensive than the standard steel parts.
I hope this list is helpful without scaring you off. Because saltwater shouldn't be scary. When you add up all the costs of saltwater, and divide by the number of useful years, you'll see that it's a minor cost.