I would use the 5 X 10 DD in the dry sand, it's a killer for finding the small gold ear rings and chains. You can run the sensitivity higher and cover more area with it, you can also use it on the wet sand as long as no water can get to it. Your ground balance will change from the dry sand to the wet sand. Take the stock coil for using in the water or at the water line. The 5 X 10 is not waterproof. You could also use the 6" in the water, it just won't cover as much ground. If you don't want to be bothered changing coils, the stock 7.5kHz coil will work just fine in and out of the water. Get there right at sun up as the others who detect the beaches are usually out at first light. I don't do much beach hunting so others may have some good tips for you. Try to find out where most of the people congregate on the beach. Try walkways also. If you don't have much luck, try some of the out of the way places that most people with detectors don't hunt as much. Find out when the low tide is and try to be there to detect at the surf line and into the water. Be sure you don't get the detector box wet or go out so far that you get swamped with a wave.
Good hunting, John K