I've never seen a puddle like that in a crypt. Very strange, maybe a bug.

Usually what I do is sail next to a swamp and look for the crypts as I go by, not so close that things attack me. You used to be able to see the green torches, but now I only see the short rectangular buildings. If I see those, I keep sailing and look for a meadow or black forest next to it. If I don't see both of those when sailing, I look somewhere else.

But your first swamp should just be any swamp that you can walk into, of any size whether it has crypts or not. Get familiar with the place, grab some loot, including bark and guck, and look for the turnips. It will take many in game days to get your turnip crop going and it's required to upgrade your cauldron.

Most of the people who talk about swamps being awful tried landing there with a boat and got the boat destroyed. Avoid doing that if at all possible. Consider the ocean biome it's own thing after the Elder. You may spend a lot of time sailing while looking for a good swamp and maybe looking for the trader. Just keep yourself out of aggro range when you're following the shore, and be wary of night, fog and storms. You are a sitting duck out there.