I cannot find the second or third glowstone, the only one that I have is the one that the mermaid gave me. I went to the treasure room like the hint book said, and in no chest is the next glowstone. I saw a hint that the second one was in the crab cage room, and cannot see it there either.

Any hints??

As far as I know, there are three glowstones:

1 that the mermaid gives you
1 in a chest far up in the rafters in a room near where you meet her...use Spirit Eye
1 in a chest leading from one of the crab cages. What you have to do is dive down one of the cages, swim through a tunnel, and then swim up. You'll be in another room where you should find a third glowstone, and another of the tablet pieces.

You only need 2 glowstones, though, to finish the puzzle to get the third tablet, so you can get by with only two if you can't find the third one. Remember to remove the glowstones from their sockets when you've finished with them; you may then reuse them.

Sazerac.