Salt water boats are more likeley to rot on the mechanical parts ( drives ,gimbals,engine and so on ) where fresh water boats more likeley rot on the wood and cores on the boat it self ( more expensive to fix) the reason of that is that fresh water carries more bacteria that eat the wood then salt water,,so on salt water boats it takes much longer for that to happen.
If u have a salt water boat that is cared for then the only thing u would need to check is the gimbals and intake,exhaust,maybe the heads if they are aluminum.
If its flushed reg.thenitshould not be a problem at all.
I would rather go with a in good shape salt water boat then fresh water.