The Shark Reef is a nice place to get away from all the busy crowds and gamblers and something to do if you want to go "underwater". Have now been there twice, but I think the cost of admission ($18 for an adult) is a bit pricey, though (and for those who are Nevada residents, it might still seem expensive even with your locals discount). Manta rays, octopus, sharks and various types of fish were great to see. Plus, a crocodile and other reptiles were featured in the first few exhibits. You'll also find two walk-through tunnels. The aquarium is on the south end of the Vegas Strip inside Mandalay Bay.
I've noticed quite a few other reviewers posting this, but there is a staff member at the interactive manta rays exhibit who has to be constantly telling people how to pet them, and boy, it seems as though he/she has to repeat them constantly all day.