I dont think the water spots are going to be an issue if he's using DI water other than that agree claying the car right after you wash it is the way to go !!
:iagree:
If you decide you only want a wax as a last step instead of both a sealant and a wax, I would give very serious consideration to the Meguiar's Microfiber System for both pads and polishes: Meguiars DA Microfiber Correction System, paint polishing system, car polish, car detailing...