Check you inventory. You likely picked up a special spell/scroll (dropped by the guys in the last cavern of the dungeon directly before the lake) you have to use/read while standing directly in front of the lake.
Clicking on the lake is pointless.
Unfortunately i don't have the spell on the inventory anymore. Anyway to add it with editor maybe?
Hm, you should have it. IIRC you can't sell it. Sure you even picked it up?
