My advanced horse does the aquatread as part of his conditioning program. On those days he does not do anything else... it really works them all over, and usually I go for a long slow hack the following day (or a lighter flat). It is fabulous though for enhancing your fitness/rehab work! My horse will be coming back in a couple weeks from a tendon injury (has been aquatreading for a month now), and he is going to be WIRED!
I've had a couple of horses swim (in a pool). None advanced but I've known some advanced horses who also went to the same pool. We treated it like a gallop day in the schedule. They didn't do anything else that day.
** The difference between genius and stupidity is genius has its limits. -- Albert Einstein **
Back when I was a working student, the facility we were at had an in ground pool. Upper level horses (this was back when we had long format!) were swam once per week. The had the rest of the day off except for a hand walk (as poster above mentioned, it’s a real work out!).
Horses still were hacked out for hill and galloping work. Swimming just added another level to their fitness. Its hard work, at first they can do just a few minutes at the most, time in the pool is slowly increased as fitness improves.