Hello JRO, I'm somewhat new to this board. I think a great deal of this question also depends on how many photos you take per session and per year. I'll get maybe 30 to 70 shots per session, and am lucky to get 6 dive trips in a year, so I don't feel the need for a dedicated light table/loupe program like Aperature, I just plod through my photos in photoshop siphoning the "keepers" into new folders as .psd files so I'm doing all the sorting myself. I can use bridge to view the keeper folders if I need to, and I also make separate iPhoto albums (yes, iPhoto :shocked2

for use with automated web gallery programs.
I think the photography vs graphics issue applies purely to "On Land" photography. IMHO the color challenges involved with underwater photography make photoshop an eventuality that cannot be avoided.
If budget is a concern, or if you shoot zillions of photos, you might get Aperture first. If your number of photos is not unmanageable and you've got the dough-ray-me, you might want to go straight to photoshop first.
As for sharks vs water, with color editing, you always move from the largest problem first to the smallest problem last. As M_Bipartitus aluded to, in underwater photo levels will almost always be your largest problem and will almost always be your first move. If your levels are fairly straight you could then move on to the shark and you could use a selective color tool to futz with the shark.
The Selective Color color control in photoshop has Neutrals, which you could use for the shark in which case you would not even have to select the shark. Hue Satch selective color control has no neutrals so in that case you have to at least make a loose selection. Selective Color in this case would be your best choice over Hue Satch, but it is a CMYK tool so it might intimidate new users and is probably not available in Elements.
Hope that helps. I'm somewhat long-winded, I know.