No rinse tank water in a saltwater operation is going to be salt free, Unless they have a continuous supply of freshwater running into it and the saltwater is being circulated out. So the chances of a true freshwater rinse tank are very small.Rinse tank water is supposed to be clean. I'd question whoever maintains it how there could be such a high salt content in that water.
Dunk one BC in it? Now a saltwater tank.
I never used and recommend against putting regs in rinse tanks anywhere. Maybe your own bathtub pressurized on a tank. Dust caps are just that. Dust caps. Don't trust them for dunking regs.
Best way to rinse a reg on a trip? Put the dust cap on, drape the regs over your shoulders and shower with them briefly.
Do that after the last dive of the trip. Rinse the seconds in the sink while working the knobs and levers if any. Swish them around. Rinse again. Wipe the 1st stage with a damp cloth. Hang them to dry overnight at least if possible. Never put wet regs in a reg bag for travel unless it's only for a couple hours.