If you do get Photoshop Elements, I strongly recommend getting the book by Scott Kelby, "The Photoshop Elements Book for Digital Photographers".
It doesn't have much on manipulating underwater photos, but it does cover lots of other subjects such as red eye removal, portrait touchups such as removing wrinkles and blemishes, and even such wild things as slimming buttocks, brightening teeth, and colorizing hair. I found his straightforward explanations of how to do such things taught me a lot about the various Photoshop Elements features that weren't obvious from reading other books and the manual.