Welcome to Treasure Net Sandhunter...........
You can make your own harness or buy one from Kellyco who sales the ones I use. I have found them invaluable, I used a soldering iron to burn holes in the harness in the chest area and mounted clips to attach hooks to hang a waterbottle and waterproof cigarette holder that I put special finds in. (jewelry)
The harness takes all the weight off your arm, you just guide the coil. I can hunt all day with mine. You detector will also hang by your side when using the harness, coil to the ground if you let go which is helpful when recovering a target in the shallow water or wet sand.
I have two Sovereign GT's, one has been custom waterproofed and can be used in the surf, the other is not and if I need to go in the shallow water I have the controls hanging from my neck in the pouch that came with it so I can enter very shallow water. Cover the top of the controls with a half gallon baggie to keep the salt spray off the controls. I don't go any deeper then calf to maybe knee deep due to the danger of a rouge wave or tripping in a hole. I have 2 Excals and the waterproofed Sovereign GT for surf hunting.
Both of my Sovereigns have the WOT coil, and I also have the new SEF 15" x 18" Butterfly EXcelerator coil from Kellyco. It is heaver then the WOT, but it also covers a lot more area and works as well as the WOT. I have 2 of the harnesses with bungees and I modtified one with a heaver bungee cord and replaced the hooks that go on the bungee with thicker ones, the old ones were wore half way threw from extended use. (preventive maintenance)
Good luck with your new Sovereign GT, you will find it is a beach vaccum when combined with the WOT or SEF Excelerator coils...