If you are going to use a large search coil for beach or water hunting you have to work on making sure you keep the large search coil low and level throughout the sweeping process.
Search coil control is important if you want to take advantage of the extra target depth provided by the larger size coil.
If your metal detector balance is thrown out of whack by a heavy search coil, you may get sloppy struggling to maintain a level sweep. Large search coils are often heavier than the standard size coil that came with your metal detector.
Using a metal detecting harness or a search coil stabilizer will help you from struggling with the extra weight and sweeping higher above the surface of the sand than normal, also the dreaded raising of the coil at the end of each sweep.
If you feel a metal detecting harness is too cumbersome, try using a sash type sling with a bungee cord attached from the sling to your metal detector shaft.
This simple type of harness to distribute the weight of the metal detector is similar to what people using garden weed whackers or pressure washing poles use.
Ground coverage is the main reason beach or water hunters use large search coils but in my opinion it should be about target depth and having the ability to detect targets at greater depths than the standard size search coil.
Walking around with a large search coil several inches above the sand because you cannot control it often negates most of the extra target depth, having to lower the metal detector sensitivity to use the large search coil takes away the rest of any perceived depth advantage.
One pulse induction metal detector I had my eye on for a while has a large search coil mounted to the lower rod at the back of the search coil.
Every beach hunter I see using this metal detector has the front of the large search coil tilted up at the front, this put me off buying and using the metal detector.
Search coil control is very important to me and I know every inch quite often counts at the beach so I prefer to use search coils that mount towards the middle of the search coil.
Search coil control is probably why some beach or water hunters find stuff over ground covered by other people, especially when using the same type of metal detector.
When all things are equal metal detecting equipment wise, its the person with the better basic metal detecting skills including search coil control that has the advantage.
A large search coil is only an advantage at the beach if it is swept level and very close to the sand.