A boat with 30 hours in the bottom of the sea won't be worth much but one that was trailered and perhaps flushed after each use could be in great condition.
You need to inspect it carefully, especially for signs of corrosion. And, of course you need to water test it, take an hour cruising around, test if everything works. Does it run well and look well taken care of? Like new, this boat should look.
Was it left in the water hours, days weeks months? Does it show signs of corrosion? Are they asking a lower price as an equal freshwater boat would bring?
Is the trailer at least galvanized, in my experience you can hear a mild steel trailer rusting away after being dunked in saltwater.
If the boat has an engine designed for fresh water, flushing is more important. Are there signs of corrosion anywhere? If so, fasteners tend to snap off when disassembling for service.