Aquariums and Animal Welfare
The animal welfare position holds that humans have a right to use animals as long as the animals are treated well. However, even from an animal welfare viewpoint, aquariums are problematic.
 
Animals in an aquarium are confined in relatively small tanks and can get bored and frustrated. In an effort to provide more natural environments for the animals, different species are often kept together, which lead to predatory animals attacking or eating their tank mates. Furthermore, the tanks are stocked either with captured animals or animals bred in captivity. Capturing animals in the wild is stressful, injurious and sometimes fatal; breeding in captivity is also a problem because those animals will live their entire lives in a tiny tank instead of a vast ocean.
 
Special Concerns About Marine Mammals
There are special concerns regarding marine mammals because they are so large and they so obviously suffer in captivity, regardless of any educational or entertainment value they may have for their captors. This is not to say that marine mammals suffer more in captivity than small fish, although that is possible, the suffering of marine mammals is more obvious to us.
 
For example, according to the World Society for the Protection of Animals, a dolphin in the wild swims 40 miles per day, but US regulations require dolphin pens to be only 30 feet in length. A dolphin would have to circle his tank more than 3,500 times every day to simulate his natural range. Regarding killer whales in captivity, the Humane Society of the US explains:
 
This unnatural situation can cause skin problems. In addition, in captive killer whales (orcas), it is the probable cause of dorsal fin collapse, as without the support of water, gravity pulls these tall appendages over as the whale matures. Collapsed fins are experienced by all captive male orcas and many captive female orcas, who were either captured as juveniles or who were born in captivity. However, they are observed in only about 1% of orcas in the wild.

And in rare tragedies, captive marine mammals attack people, possibly as a result of post-traumatic stress syndrome after being captured from the wild.
 
What About Rehabbing or Public Education?
Some might point out the good work that aquariums do: rehabbing wildlife and educating the public about zoology and ocean ecology. While these programs are laudable and certainly not trivial, they cannot justify the sufferingof the individuals in aquariums. If they operated as true sanctuaries for individual animals who cannot return to the wild, such as Winter, the dolphin with a prosthetic tail, there would be no ethical objections