My impressions on that lake are that when you aproach it from west running on the coast touching the water - it hurts, but damage is not high.
After I turned into ghost form and flied to the east coast - there is that Black ring cave entrance I went to the coast and touched water again. I died as soon as I touched it.
So, I assume the further you go towards eastern coast with the cave, the more damage you get from that cursed -poisoned lake. I think, no way to cross it in human form.
