I myself am not opposed to zoos or aquariums, but those are first and foremost institutions of learning. Seaworld is a for-profit corporation whose sole focus is to provide entertainment using animals in order to make money, so that needs to be factored into any discussion. And point of fact, many so-called activists don't like zoos or aquariums either.
I don't think it's "trendy" for people to go against SeaWorld or other similar places. I think people are simply becoming more aware of what captivity can do to certain animal species and are disagreeing with SeaWorld's philosophy. After I saw Blackfish, I did some minor research on the "charges" against SeaWorld. While I couldn't attest to the accuracy of some of the claims (i.e., policies of separating whale families or treatment of care, etc.), there did appear to be some misleading information coming from SeaWorld as it related to the natural health and behavior of Killer Whales in captivity versus the wild. Longevity, in my research was mixed. Male whales didn't seem to be too affected by captivity versus the wild, though female whales appear to live longer in the wild than in captivity. SeaWorld was slightly misleading in this claim, using the data of official research out of context and ignoring outliers (natural conditions changed for a period of time during the research period, resulting in an anomaly of lower lifespans in whales for a short period). The most egregious claim was the collapsed dorsal fin, which rarely occurs in the wild and to which SeaWorld still claims is entirely natural.
Since sea animals do not share a domestic coexistence with humans in the way dogs or cats do, and to a lesser extent horses, forcing them to perform in shows (and this goes with elephants, lions and tigers and other wild animals) is folly and abusive, no matter how well-cared for the animals are. And I think the population at large is coming to realize that more and more. Seeing an animal in a recreation of its natural habitat is more palatable than seeing it jump through hoops or balance a human on its back, which serves no educational value. I think SeaWorld was somewhat on the right track with the expansion of its whale tanks.
I did find it rather cool to see a killer whale, but SeaWorld should have evolved faster with how to display them. Now they are losing their biggest draw because they were too busy defending themselves against a documentary as opposed to finding was to establish with the public their dedication to preserving the health and well-being of these animals.