One of the easiest ways of increasing the depth your metal detector can detect targets at the beach, is to think about how big your search coil is and how far above the sand you are sweeping your search coil. Notice how I refer to a sweeping action, not a swing like using a golf club.
From this site
If you are using a 10 inch search coil on say a Minelab Excalibur, you should be able to detect any of the platinum and gold bands in this photo at a depth of 10 inches.
If you walk along the beach sweeping your search coil two or three inches above the sand, you are going to lose 2 or 3 inches of depth and miss valuable targets.
Swing your search coil like its a golf club, with luck you may hit targets 7 inches deep along the center of the line you are walking along.
Not having your search coil level and as close to the sand as possible, means you are not taking advantage of the size of your search coil.
A good DD search coil should be able to detect targets down to the same depth as the width of the search coil.
A good mono search coil should be able to detect targets deeper than the width of the search coil.
Every inch you hold your search coil above the sand is an inch less you are going to detect a target under the sand.
In a game of inches, why would you not want to use the full detection range of your metal detector search coil.
Your beach hunting technique automatically improves when you concentrate on keeping your search coil just skimming the sand.
It slows your walk (detecting pace), slows your sweep speed, which in turn puts more platinum, gold and silver in your finds pouch.
Adjusting the length of your metal detector shaft and making sure your search coil is level by keeping the bolt tight will also help with target depth.
Search coil control is not mentioned very often, but it can be a difference maker to a beach hunter.