If you have a swim up bar, a raised bond beam counter or a ledge that meets a bar top, in-pool barstools are what make it usable. These sit in the water and hold their position, so swimmers can pull up to a counter without treading or standing on a step the whole time.
Getting this right is purely arithmetic. Measure from the floor of the pool, ledge or bench that the stool will sit on up to the underside of the counter, then leave roughly 10 to 12 inches of legroom between the seat and the counter surface.
Remember to measure to the finished counter surface, not to the top of the wall before the cap or tile went on. That detail is the difference between a comfortable seat and one nobody uses.
Marine grade, UV stabilized resin means no rust at the footrest, no corroding fasteners and no peeling finish. The stools can be ballasted so they stay planted when someone leans on the counter or a swimmer brushes past.