Erk! The swamp is quite a step up in difficulty from the black forest! I found a swamp biome next to a meadows biome on an island. Perfect - temporary base in the meadows biome, forays into the swamp to look for crypts to get iron. Off I go, poison res mead and my normal food mix of meat, berries and mushrooms and...aaarrgh, everything is trying to kill me! There's venomous things in the trees, venomous things in the water (and there's lots of stretches of shallow water - it's a swamp), skeletons all over the place and lots of draugr. Who are much, much tougher than greydwarfs. I saw some things on fire wandering around a jet of fire coming up out of the earth, but I decided to not go there.
I found a crypt and opened the gate with my swamp key, but the entrance was blocked by what looked like some sort of tree but I couldn't do any damage to it. It wasn't even showing 0 damage. No "too hard" message. Nothing. Maybe it's because I killed Elder in a different world to the one I'm in. Maybe it's because I forgot to put Elder's head on the stone.
I got a bunch of new things to craft with, though. Mostly horrible things, like entrails and stinking ooze. But not iron, which is what I really want.
Yes, I am just a few days ahead of your exploration I think, and I also found the swamp very "erk"!
You definitely need better food than berries and mushrooms for the swamps. After my first foray ended in poisoned death and then my next ones ended with me bravely running away, I decided I needed to up my survivability a notch.
Entrails can craft sausages, you should have access to carrots and turnips about now too, and you can make jam. Fishing is also good. My current swamp expeditions are fueled by cooked fish, carrot soup/turnip stew and sausges. Jam or soup if I am running low, but nothing less than that. Having a small stack of healing potions to hand for emergencies is a must (usually when I get poisoned when already at half health). With a poison resist mead up this makes the swamps tricky but not overwhelming so long as I take it slow.
Some other things I have discovered that were not immediately obvious (to me at least):
You can hit leeches safely from land with the AoE of the stagbreaker or (my preferred method) the alternate attack of a mace (I only discovered this yesterday and it has made a massive difference for me).
I can usually get two mace swings in on a blob just as it lands before it can do its poison attack. Provided I don't miss this should kill it safely.
Draugr can be parried easily. I was farming these for trohpies at a spawner last night. Kite, parry and hit. So long as you don't get greedy with the parry follow-ups they become perfectly manageable. You can even parry arrows and get the stagger effect and parry attack bonus (I expect this will be patched out at some point), and the parry window is really generous, so raising your shield when you see the bow being drawn is usually enough.
My personal favourite tip that suddenly occured to me after pathing between bases: you can hoe the swamp! I made a nice ridge of dry ground for my route to the crypts areas, which means I can back up without having the issues of being in the water.
I still run from the flying deaths, though. They hit like a truck!
I found another biome near the edge of my swamp and was chased back across half the island by a flying insect that took me down to 4hp with one hit. Fortunately it also finds greydwarfs a tempting target it seems, so just as it was looking hopeless I was able to get some distance and lose it whilst it wiped a dwarf pack. I am not going back in that direction in a hurry!