Rumors tend to have some truth to them. From what I was told (from GA marine mammal trainers that were there early on), the idea came from Bernie Marcus himself. He had a strong conservation passion for them and thought he could possibly breed them

hence the original design was to hold large marine animals, not necessarily whale sharks. Someone convinced him not too (the logistics alone are mind boggling) and whale sharks were chosen instead. The reality is it was never going to happen, a billionaires dream.....
Sidenote: This, according to former staff, also caused a ripple effect. With the whale sharks being chosen it was decided to add other large sharks that originally were slated to go where belugas are. By not having the current beluga exhibit be a large shark display and the facility wanting ceataceans of some sort, that's how it became belugas