Shut the motor down when the tuber is ingressing/egressing the water. If you have to reposition the boat near someone in the water, use a paddle (or two), or toss a line to the swimmer and drag them where you want.
Make sure the observer knows their job is observe the towee; not just to warn when they're down, but also to watch where they went down, so you can get back to them straightaway. Make sure the observer meets any age requirement if there is one (pretty common in the US.)
Be really careful if operating around fixed structures (rocks, pilings, barges, piers, etc.) The towed person is ikely to come off at the outside of the turn, and may skip a bit across the water. Keep enough separation so they don't skip into something hard and unyielding.