I had the same issue as the "Unreachable key in Lakebed Lair" #88 , but was not able to go back into the dungeon after completing it, since the water in the lake filled back up. Upon returning into the sewers, the button to drain the lake was already depressed. I have flippers and blue tunic but these do not appear to allow me to dive and re-enter the dungeon.
I had the same issue as the "Unreachable key in Lakebed Lair" #88 , but was not able to go back into the dungeon after completing it, since the water in the lake filled back up. Upon returning into the sewers, the button to drain the lake was already depressed. I have flippers and blue tunic but these do not appear to allow me to dive and re-enter the dungeon.