See
this link for more on the pseudoscience of magnetic water treatment for preventing scale, etc.
The product you linked to is completely different. It is an ion exchange resin, basically what most water softeners use. It exchanges calcium and magnesium with sodium. It is very likely to work and is a reasonable way to reduce the hardness of water. Just note that in a spa you usually do want SOME water hardness to reduce foaming. Usually 120-150 ppm CH is sufficient for that purpose. You don't want much more since you want to reduce the risk of scaling.