TAMU-Galveston's mission is centered specifically on maritime and marine science programs. Their marine biology program is in the Top 5 from a curriculum standpoint, but something like #10,000 in terms of location. It's hard to stay focused academically when your local study site is icky Galveston Bay and the upper Texas coast, and your campus is a near-treeless plot of dredge spoil overlooking the Galveston ship channel.
Their marine science program is well regarded, but not designed as a terminal degree per se. It's more of a prep course for budding oceanographers and technical type (GIS scientists, marine techs, facility managers). You have the option to learn a little marine biology with this major, or a moderate amount. It's far less involved in fieldwork than the marine bio program.
TAMU-Galveston's marine engineering and maritime systems engineering programs are top-rated... graduates have no trouble finding jobs. If you enroll as a merchant marine cadet, you'll spend a lot of time working aboard the training ship (currently the Sirius), and go on at least one 10-week summer cruise.
Neither TAMU, TAMU-Corpus Christi, or TAMU-Galveston offer undergraduate oceanography degrees. It's a graduate degree at TAMU only, although you can take undergraduate oceanography courses at TAMU-Galveston. If you're a marine science major, you're actually required to take introductory oceanography or its equivalent.
If you like nature study, you'll get far less exposure in oceanography than you would in marine biology. Oceanography places much more emphasis on chemistry, geology, and technical knowledge. Only the sub-discipline of biological oceanography focuses much on critters, and not too much, at that. But the oceanographers get first dibs on all the fancy research cruises!
Plenty of oceanographers with Master's degrees get jobs. State and federal govt. actually prefer Master's degrees. A heap of consulting and monitoring firms snag Master's grads. PhD's are primarily confined to academic institutions.
TAMU-Corpus Christi is the only TAMU school with a decent diving program (excepting the nautical archaeology dept). But really, if scuba is something you really want to integrate into your college degree, stay out of Texas! Florida is the place to go for this in the South.