Gary,
yes it can be used in wet sands - BUT you will have to make manual adjustments to the M6 in regards to the sensitivity. Please spend some time reading and becoming completely familiar with pages 14-16 of the M6 manual. it discusses the proper sensitivity settings and how to make the mine adjustments to the M6 that will offer you the best results for the ground that you are detecting. There is also a threshold adjustment that can be made to the M6 but it does require a box split to do so. Try working with the sens setting first and if need be do a Google search "Whites M6 threshold"
Hope this helps.