You need to get fairly close to the lake, then switch to ghost form. Level 1 of the spell should be good enough, as long as you don't waste too much time stoping or wandering; I started pretty much straight west of the map flag for the dungeon entrance and went straight east over the lake, and didn't have any problems.
BTW, there is a little bit of a buffer area at the west edge of the lake where you will just take damage, but on the other side it is instantly deadly. Apparently in games started after the patch is installed (or in re-release versions of the game already updated) there is a bit of a grace period, so even if you transform back in the middle of the lake you can pause the game and then transform back to continue.
Alternately, you can toss a teleporter stone across the lake (hold the shift key down to move the screen focus over with your mouse cursor) and get there that way.
