Swamp is a big transition. Mountain is really more of a step down in difficulty IMO.

Poison resistance mead is very easy to make, though. All the ingredients are easily accessible, except honey if you haven't made a bee hive yet, but that's not hard and necessary for many other things.

I have never harpooned a sea serpent, so I don't know how hard that is. I just shoot them with a bow, or (mostly) just ignore them. Their meat is good, but I haven't found a use for the scales, other than to make a shield that sits in a chest. Maybe I should put it on the wall somewhere...