It's hard to tell from the photo if it's an actual low spot or just an area where the water is draining from higher up. It would still be a place I would check. If the sand is hard there, it could be very good. If it's mushy, you're probably only going to find light junk like pull tabs and foil. If I didn't know that beach, I would start by going from the top of the slope by the dry sand, and head all the way into the water in a zigzag pattern until I could determine where the hot spots are. Then work parallel to the water following the "treasure trough". What machine are you using now?