My opinion, there's a major ethical difference between keeping a marine animal in captivity, as opposed to a land animal. In a good zoo, animal life expectancy typically shoots up astronomically, thanks to individual care, a regular and healthy diet, immediate access to medical attention, and the rarity of intra-species infighting. A lion would be very lucky to make it to ten years in the wild, and they hardly ever last any longer than that - in captivity, that number doubles. Good zoos also do what they can to keep their charges mentally engaged and stimulated, and the animals often form bonds with their keepers and enjoy their company.
But with sealife, an aquarium is no substitute for the real ocean. Animal lifespans are shortened in captivity - the conditions of the water aren't what they're adapted to, and their health fails as a consequence. Often this is due to toxins like chlorine, sometimes it's an absence of microfauna that perform some obscure function in the animal's body, and sometimes it's stress. These are animals which are often adapted for migration: the vast majority of zoo animals are territorial, or are content to stick within a small area if food and social conditions are acceptable, but a fish or cetacean can travel thousands of miles in a year. Confinement does bad things to them, and there's no tank big enough. Captive cetaceans are prone to psychosis, which is why keepers are occasionally attacked and killed - something that never happens in the wild unless the animals are provoked. In the case of sharks, they have such a ridiculously sensitive battery of senses that a captive environment is simply too much stimuli, and puts them into a constant state of stress. This is why great whites fare so poorly in aquariums.
