Personally I never saw any instance of rescue guys having to pull students, but the Naval aviation groups I was with had already been through earlier training/testing and students with issues were weeded out early. Nonetheless, the rescue swimmers were always there. They wouldn't even begin the real testing before I demonstrated an ability to meet certain benchmarks. They are very careful, but I have no doubt there have been accidents.

The last time I did this I was literally the only "student"; there were four instructors on the sides evaluating, with two rescue swimmers and me in the water. Without the lines created by having lots of students, it was all over in an hour and was a real work-out. In prior quals there was time between tasks to catch your breath, but it took all afternoon. In the pool at 8 AM and done by 9 AM was a vast improvement and probably more realistic.