I think I can help you.....
I've used my M6 on the beach many times with success. I have found clad and silver coins, jewelry, toys, and a lot of pulltabs!
If you're hunting a white sandy saltwater beach, start by ground balancing in the auto track mode and turning your Disc down all the way to 0. You should be able to turn the Sens up to close to Max without a lot of chatter. I hunt in the auto track mode in the dry sand. If you use the beach mode in dry sand you may lose some depth and sensitivity to small gold items.
In the wet sand it's a little different. With the same settings above, you'll get a lot of falsing from your machine. In these conditions, you'll need to dial your Sens back until the falsing goes away (sometimes to about halfway). Ground balance in the Beach mode over the wet sand. Keep it in Beach mode and you should do fine. Also keep the Disc at zero.
The stock 950 coil is great on the beach - good coverage and depth. No need to use a smaller coil unless there is a lot of trash on the beach.
I'm not much of a water hunter so can't help you there.
Good luck!