yep theres only 1 trick that will really work ---lower the sen level to about 2 or 3 bars ( go as high as you can go with it remaining stabile)-- you will lose depth true but it will no longer false like mad (so the machine will now be usible)-- the 250 being a preset ground balance type machine --"sees" the minerals in the sea water (wet sand) as "metal" and reacts to it accordingly --beep, beep, beep --you have to lower its touchiness level by dropping its sensivity / power output level until its stable.
this trick also works in highly mineralized ground inland as well --as well as if nearby power lines " bleeding off energy" are causing issues .
good luck -- ivan