I love metal detecting in the water whether it is at a beach or a lake. But, sometimes water conditions make my pin-pointer almost useless.
Under ideal conditions, metal detecting in the water at the beach is the best. If the surf is flat, and the water is clear, detecting is easy as you can see your detector coil and know exactly where to scoop your target.
If you are metal detecting in rough surf, then it will be difficult to spot your coil in the water and you will not know where dead center is making pin-pointing difficult.
The same can be applied if you are detecting at a lake or reservoir with a muddy bottom. Walking with your detector will stir up the mud make the crystal clear water a muddy cloud.
Here are a couple of ways to work through this pin-pointing problem.
You can just try to find your target by guessing where you think the center of your coil is. This may work but you will most likely spend way too much time digging and scooping. You can pin-point a target and then slowly raise your metal detector and try to identify the approximate area where your target is and then start digging. My favorite way is to use my toe. When I pin-point a target, I slide my foot forward to the edge of my coil. Then, I move my detector out of the way and scoop the dirt directly in front of my toe. This is not 100 percent accurate but it is fairly reliable.
Metal detecting in crystal clear, calm water is the easiest way to detect but conditions are not always perfect. So, try my toe method and see if it improves your finds.