From what it looks like in the picture, that empty space has access to a ramp which leads to the main service road backstage. I'm not sure what they would be pulling in and out of the water from that ramp, but whatever it is, a second ultrasaur probably would've blocked it off.
Or, that ramp was originally intended for service to animatronics and it really was just a budget cut. It seems weird that they would design Orlando's River Adventure to purposefully be inferior to Hollywood's.