Yes.  Even though we do not measure Salt in Salt Water Spas, the Purezone does work in salt water spas. 

If you have a salt water spa it is very important to watch the FC level which Purezone measures on a daily or every-other-day basis. In salt water spas, instead of adding chlorine to sanitize the spa, you add salt, and then a chlorine generator converts the salt to chlorine. 

FC levels for salt water spas should be maintained between 1 and 3ppm.  pH levels should be kept at between 7.4 and 7.6ppm.