I've used the 705 on the wet salt beach. The 705 is the step UP from the 505, as you know. The 705 SUCKED on the wet salt. Was only getting half the depth that I was/am accustomed to getting with my Explorer. And yes, staticky performance. Had to turn the sens. way down. So the "beach" mode they got on there is a joke. Yes it will make it work, but your depth is half of what you'd get on other machines.
This is because, since the 705 (and I suspect 505) were meant to be cross-over machines, for both nuggets and/or coins/relic, that you will therefore have only a so-so compromise in each dept. This has been the case of mostly all detectors which attempt to be a cross-over machine. Is that they end up with compromises for each venue.
I ditched my 705 as fast as I could. Sorry to be a kill-joy.