I'll grant you that the sparseness of crypts on many maps is REALLY irritating but you can't drown in Valheim and they're called "sunken" crypts so yes, you can reasonably expect some water in there. lots of good tips here but i'll add one that oft goes forgotten and is especially true in the swamp:

Walk, Don't Run.
or at least, don't run until you HAVE to. This goes for jumping as well. the wet debuff combined with the nature of the terrain in the swamp requires the player to fully manage their stamina effectively or die. it's the first time the game throws this at you, so it's tough. most people run full time in all games (hell, I do) but Valheim punishes you for it in a number of places and ways. best lesson the swamp taught me.