How do I get into the lighthouse?

Hint 1 of 12:

Did you notice the key on the floorboards?

Hint 2 of 12:

Well, that's not the key to let you get into the tower.

Hint 3 of 12:

But you might want to try draining the lighthouse.  See the first question
in this section for more help with this.

Hint 4 of 12:

That chest below might have something important in it.

Hint 5 of 12:

Unfortunately, there's no key for the chest.

Hint 6 of 12:

Or perhaps there is.