IMHO, proper freezing is indiscriminate to f/w or s/w water parasites. It kills them all.
Saltwater pathogens - bacteria, virus, and parasites - appear to be pretty hard to be picked up by a f/w fish. It probably happens but I imagine rarely or nearly impossible when common sense precautions are followed.
I catch ~20 cubic feet of marine fish a year and freeze it, I believe correctly, and feed it to my f/w fish. Have been doing it for 5 years. Not that I have researched it with a microscope and stuff but so far I could not pin down any problems I've had to pathogen transference from the baitfish.