I do agree that Sea World's theming has been hit or miss. Aquatica's well themed and although it lacks the scale of Volcano Bay, it's a more interesting concept. They did try to create more themed areas. The bit with Kraken and Journey to Atlantis would have been a mythical themed area. The Antartica area has a strong theme, and I guess the Seasame Street area does, although I haven't seen it myself. I don't actually have a problem with the entrance to Ice Breaker, but looking at the POV onride theming is lacking. I think it is just a case of the budgets being tight.
There are two problems here. The first is that in the Orlando area you have to invest a lot to stand still. Arguably the gap between what Disney/Universal and Sea World are doing is bigger now than it was when Sea World was doing things like Turtle Trek, Journey to Atlantis and Antartica. Of course, you could argue that Sea World can't outspend Disney/Universal so they're better off doing completely different kinds of rides, which is what they're trying to do. Although Velocicoaster and Tron coaster are both much bigger high thrill launch coasters with a lot more theming.
The second problem is that the public appetite for animal shows and captive whales/dolphins is declining. As the interest declines, they need to put more emphasis on the rides and theming, which is difficult to do with their current budgets.