Когда огненного уже замочил (т.е. не раньше), нужно вернуться в Venetia и оттуда доплыть до North Island. Там будет пещера.
Спойлер
2.3.2 UNDINE, THE WATER ELEMENTAL
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Return to Venetia and now the boat that offered you passage to Demitel's island
has another option, this one to the North Island. Take that option and go into
the cave.
Here you have to flip 2 switches in a certain pattern for the water to rise and
fall. First, head right and hit the switch in that area, then continue on left
and head down to where the water was for a few chests, and continue going down
until you reach some stairs, go down and flick the switch, Go back to the
original switch and push it back up again. This time head up to where that
locked door was which is now opened and drained of water. Go through and head to
the northern most part, and hit the farthest right switch. It drains the water
to the path to hit another switch and so on until you find a hole in the ground
in the top room.