Yeah I did wonder that. Dunno how far Dolphins echo location is effective from. But presumably it's further than most people probably can see underwater. Especially if the water is murky or dark. It could allow the dolphin to detect the diver beyond the perception of the diver. Then presumably the dolphin is trained to rush the diver and 'bump'/smash into the diver before he has time to react or even see the dolphin.

Still does all sound fairly risky for the dolphin. Think I'd still rather be the dolphin given it's much better suited to water than humans and probably has the perception advantage with echo location.