Texas A&M Galveston is top-rated for marine mammals, I think it may even BE the top. Like all universities, what determines a good program is the faculty. And we have Bernd Wursig.
http://www.marinebiology.edu/Faculty/Wursig.htm
And Randy Davis and Markus Horning. They do those "seal-cam" things.
http://www.marinebiology.edu/Faculty/Davis.htm
http://www.marinebiology.edu/Faculty/Horning.htm
But like most marine mammalogists, a lot of the marine mammal-type graduates end up working with dead critters, or taking tissue samples from live ones. Very few work with live animals in behavioral or interactive studies. For that type of work, zoos are the place to be.
There's near-zero scuba opportunity with marine mammals, unfortunately. Piggybacked cameras are the current method for filling in our behavioral gaps, and they work pretty well. There's also acoustic monitoring.
TAMU-G has a pretty high dropout rate in the first year, with a very heavy weighting from the folks that come in there "wanting to work with whales and dolphins". Once a lot of these folks find out how much work is involved, and what the field is actually like, interest wanes rapidly.
Research your options very carefully, and avoid getting anything from TV or movies. Check faculty's online resumes, and see what their
active research is. Then see what opportunities there are for undergraduates. Most faculty research never involves undergraduates. TAMU-G has a very popular volunteer program with the Texas Marine Mammal Stranding Network, and the Moody Gardens Aquarium. This is an excellent option for undergraduates (or anyone!) to test the waters. It permitted me to swim with four different dolphins (3 adults, 1 baby), and evaluate/dispose of carcasses on beaches. Some of my best college stories!