Spent an hour at Underwater world early this year. It is a relatively small place but you still want to enjoy viewing the fishes, and not just busy snapping pictures. Dolphin lagoon show is about 20 min or so. Didnt take any picture as I didnt bring zoom len.
Depends on how patient you are or want to be. Went there last month and spent a good 1 1/2 hours in the acquarium slowly taking my time to get better shots. The arapaima exhibit is wonderful, so is the jellyfish. The coral eels located immediately after the entrance makes for good shots too.
Dolphin exhibit, yes, bring your zooms along. A bit of anticipation is recommended cos the presenter gives little lead time between announcing the stunts and it taking place. But the shows tend to be in the afternoon, so the lighting isn't great.