I hunt with a friend who's machine did exactly what yours is doing last week. His problem was his headphone connector into the box. It apparently had loosened up slightly and was causing what yours did. It worked on land but once the resistance of water got on his cables, it caused the machine to go crazy. Check all your connectors and make sure that they are all tight and completely sealed. Also, do you regularly remove and clean your coil cover. A lot of people don't do this as regularly as they should and stuff gets trapped in there (this one is unlikely, but I just thought I'd mention it). Lastly, what is your sensitivity set to? The water your hunting in might be your problem, maybe you have to lower your sens more to get stable in that water even though it might be fine on the beach.
So, check all your connections and let us know about sensitivity setting and maybe that will solve your problem. If not, post up again and tell us what its doing and maybe we can figure it out. Hope this helped.
HH -Marc