You don't need to explore a swamp to have a good idea if it has crypts in it or not. Narrow swamps (where you can see another biome behind the swamp) likely don't have crypts. Big crypts will likely have plenty, and you can almost always see at least one, if not several from the water.

Just sail around until you find what looks like a large swamp, then sail along the edge and see if you see any crypts. If you don't see anything, move on.