Originally posted by L3EEk:

Additionally, every swamp will rain regardless of weather conditions outside the biome. If you're planning on staying there for long periods of time, I recommend making yourself some poison resistance mead. A lot of the enemies in the swamp can poison you, be careful!

Personally I'd recommend just having tasty mead on you. Poison isn't a factor as long as you stay out of the water while leeches are around.

Though you will definitely need poison resist to fight bonemass.