Most, if not all saltwater damage is in the form of metalic corrosion. The boat itself isn't too much affected, however washings and waxings should be performed more frequently. Salt will accelerate gelcoat oxidation and deterioration.
Saltwater damage is really no worse than freshwater damage other than the damage is done a lot faster! It consumes more time and maintenance to prevent damage and prolong the life of your equipment.
Just like anything else it comes down to how well you maintein your gear. Replace zincs regularly, flush religiously with some kind of "salt-away" agent and always have a fresh coat of anti-corrosive spray on everything in the engine compartment.
Lakes and rivers also have minerals and ionized particles suspended in the solution, the only difference is that it is much weaker.
Put an iron bolt in a bucket of saltwater and lake water....both will rust away in time, the salt just faster.