There have been MANY threads on the marine biology topic and I highly encourage people to read them. I for one do not recommend an undergrad degree in marine bio; general bio with a broad background is often more marketable long term and gives you a better foundation in a HIGHLY competitive environment. Besides, you may learn that marine bio really isn't all its cracked up to be and you really prefer biochemistry.
IMO, fisheries biology and aqua/mari-culture are two major areas where work is probably likely. Biochemistry related fields, using marine organisms for research into medicines, and technological advances is also a likely candidate (but get the degree in biochem). Many other fields (marine ecology, coral reef biology) are highly competitive and there are a lot of idle researchers, IMO.
If you want to open an aquarium, I personally think that Marine Biology has very little to do with it and would suggest you pursue a degree in animal husbandry, zookeeping, etc esp at a hands on training college. In fact, in the aquarium industry, many look down on those with marine biology degrees. There is little correlation between marine biology and captive husbandry of animals. Zoology, veterinary medicine, etc are IMO better undergraduate (or graduate) avenues if you want to get into rehabilitation.
Dolphin trainers are not frequently fully trained marine biologists (in fact if you say such a thing to a marine biologist they are likely to roll their eyes at you). Again, animal husbandry or even animal behavior are far better choices, IMO, in that field.