Nickel test in the salt isn't a great test nor are nickels in general. Just because they may have similar phase angle as gold nickels are an alloy and have a very strong response on IB machines.

A better logical test would be a thin gold chain in the wet salt sand and see which does better. In that scenario because small and thin gold will have similar phase angle as the salt itselft it will disapear or the machine would have to be run so unstable you would not hear it at all. The PI are immune to salinity and would possibly see it depening on pulse delay and coil chosen.

Showing coins in general for people to judge a machines salt ability is a little odd in general because who in their right mind is going to spend $5-$10 an hour to park at the beach and bring their $1500-$1600 machine to dig pocket change?

Just saying