I don't want to attempt a 10,000 word essay on the troubles of Sea World Orlando, but in short I think they've been hit by a lot of problems:
1) Increased competition from Universals, which is going to get tougher this year once Aquatica's competing with Volcano Bay.
2) The fact that Sea World missed the boat on adding its own accommodation, although there are partner hotels and plenty of accommodation along International Drive.
3) The fact that losing the tie with Anheuseur Busch and the free beer samples affected visitors more than they expected.
4) That tastes in theme parks have generally moved away from non-fiction themes (look at Epcot or Universals). Sea World tried to move towards fictional themes themselves, but this watered down the educational message which created new problems.
5) That until recently the park hasn't been divided into particular distinctive themed areas and has felt a bit samey (Antarctica has a much more distinctive feel than the older areas).
6) That some of the older rides have started to feel quite dated (particularly Wild Arctic).
7) The slow down in Brazil's economy has reduced the number of visitor from South America.
8) The fallout from the Tilikum incident which meant that trainers could no longer get in the water with the whales.
9) Cost cutting in response to falling visitor numbers.
10) Growing concerns about the ethics of keeping certain kinds of animal in captivity. This is partly because of Black Fish, but aside from this one films there was already a changing public perception, which is why we had already phased out cetaceans in the UK.
Each one of my points is controversial and could be debated in itself, but all in all there could be quite a few theories for why visitor numbers have fallen. Clearly they're doing their best to turn things around with more rides, putting more of an educational slant on the shows, emphasising conservation, dividing the park up into clearer areas etc. But to really turnaround Sea World is going to be a slow and challenging process.