+1 for Krakatau. It's the only one I've ever been on, but I thought it was a blast and certainly unique. Of course it helps that the park was nearly deserted and we got to ride back to back to back.
I also agree that the Wild Thang/Rampage area is criminally underused, but from memory, perhaps the terrain there would be an issue for an aquacoaster? IIRC, it pretty hilly, or maybe there's a ravine or something. It's a definite uphill walk from the exit of Wild Thang.