Try Ground Balancing in the water. Deep enough that the coil stay submerged while pumping the coil up and down. You may have to adjust manually, and GB often when moving to different parts of the beach, i.e. moving from dry toward the water. I GB and hunt parallel to the water, move toward the water a little, GB again, and hunt parallel again. Some places I can see the difference in the layers and know to GB in each. You shouldn't have to change much, if at all, in the water.
Like stated previously there are beaches that are very difficult to hunt. I have one that has a lot of black sand, and another covered with metal chips of many sizes.