In the Illuminated Order, I'm on the quest where you need to retrieve the remains of Imaginos, the tablet on the ship, and the log on the island. These are the items I have already:
Skull of Imaginos
The Ship Log Book
The note mentioning for the wards not to be breached on the island

What item(s) am I missing?

You also need the tablet. If my memory is correct it's in the shrine of the blue mollusk or something like that.
btw, in game it has the same mesh as ectoplasm, so it might be a bit hard to find underwater.

Thanks, but where would that be located (within the shrine)?

Somewhere on the floor, I think it's near one of the colums.

Oh ok, thank you!