I have a little 4 oz lead fish weight with a string knotted every foot. All of $.70 cost. When I get to the boat at the charter base I drop it over the side just forward of the mast and count the wet knots. I use that to compare to the depth sounder. Usually I find it set about a foot or so below the keel.

The other thing I do is find the knot log and clean it. 9 times out of 10 it is fouled with barnacles or other crud.