If you're going to use it this boat in salt, then you'll want to look out for an outboard that is set up for salt water. Most of the manufacturers have ocean and saltwater models that use parts that are more corrosion resistent.
If you're going I/O, you'll most likely want to look for a boat with a closed cooling system that is set up for ocean use.
Keep in mind that boats used in salt water generally depreciate faster than ones only used in fresh water. So if you're buying one used, salt water models are generally cheaper and if you use a boat in salt, it will be worth less if and when you decide to sell.
As far as the trailering rules go, unfortunately, I don't live in Florida so really just can't help out in that regard.
-Chris