I am very thankful to the Vancouver Aquarium for much of their ocean conservation work and rehabilitation efforts where animals are eventually released back to their natural environment. I think that the contentious issue for many is the permanent "captivity" aspect, and animals that are deemed "non-releasable" (sustained injuries that put them at a great disadvantage in the wild, or lack the life-skills required to survive on their own - born in captivity or stranded at a young age).
Tanks are great as a temporary, rehabilitative measure but it's not for us to high-handedly decide to hold captive creatures that can typically swim great distances and dive to great depths - we couldn't build a tank big enough. Since it is largely humans that create the displacement, injury and impact to these animals and their eco-systems, a better use of our time, energy and resources would be to lessen our collective environmental impact, not ‘study' the impact of our wrongs at the symptom level. The oceans and the life within is our human responsibility which requires long term planning, not a facility to house those harmed by our own short-sighted inadequacies.
As far as the "education" argument, I think renowned marine biologist, Jacques Cousteau sums it up best when he said "No aquarium, no tank in a marine land, however spacious it may be, can begin to duplicate the conditions of the sea. And no dolphin who inhabits one of those aquariums or one of those marine lands can be considered normal."