Now I understand what you mean. It wasn't the knob but the cap on top of the knob. a friend of mine did have that happen.
He fixed his by turning the knob counter clockwise to the off position or as far as it would turn to the left and then marking that position on the knob with a tiny dab of water proof white paint.
You can replace the knob by calling Minelab and explaing what happen, I bet they would send you a new knob or the cap for free, or go to radio shack and get one there. I prefer the ones that were replaced on one of my units, knobs being a little larger they are easier to adjust when wet in the surf.