Welcome to beach hunting with the Manti. With my Manti with M15 coil in seawater, I noticed that if I run Beach Low Conductor that I can't set Sensitivity much past 19 or so w/o excessive chatter and large center dot (from saltwater) at 1-2. However, Beach Deep and Beach Surf/Seawater allow me to increase Sensitivity to 24ish w/o much chatter in the surf.
Also, once you settle on a particular tone/pitch be aware that you have to set it for each Search Mode. For example, I usually search in Beach Low Conductor, but switch to Gold Field/40khz when faced with excessive EMI. But the default tones in Gold Fields drive me nuts and so had to change tones on Gold Fields to match my Beach Low Conductor. fyi