hyposalinity works, but i dont see how freshwater dips work. after all the white spot is caused by a parasite in the skin of the fish, not on it. so just dipping a fish in FW just doesnt seem to be an efficiant way to treat it in my opinion.
hyposalinity will kill the parisite in its free floating larval stage, so before it can find a host and burrow in (causing the white spot). when my tank had marine whitespot, i put all my fish in a QT tank for a month with an SG of 1.012. i didnt use any meds as i had a puffer fish, but the ich was gone after a week,
you need to let the parasite die in the main tank though, as it can lie in wait, basically hibernating for up to 90 days in some cases. its normally conserably less.
inverts can stay in the main tank, as they are uneffected by the disease. but can harbour the hibernating parasite, so your tank has gotta be emptied of fish unfortunately. as you cant treat the main tank with any copper based meds as you will kill all your inverts, including all the denitifying bacteria in your LR.