No definitive answer, I'd say generally people like to frolic in water about waist to chest high. Others make a point of it to swim out as far as allowed if a swim line is established, often clinging onto the markers/buoys or line itself.
Both can be productive but these areas are also the most reachable for competition and are often hammered clean. Some guys go out at the crack of dawn nearly every day, some towards the night, others in the heat of day [me]. So finds/replenishment will depend on the amount of traffic the beach has, and the amount of other guys detecting it. I've detected beaches with decent traffic were I haven't even found a bottecap from ankle deep to neck deep, a sure sign of hammering by guys who pick up litter [in their own interests, they don't want to keep finding the same bottlecaps]. But there are always fresh drops, and some beaches are so vast that the whole wading area cannot be effectively covered even by crowds of regulars.
I went to a beach the day after labor day, this beach does not allow detecting between memorial and labor days. Got out there at 6am when the park opened, as it was still dark I went to McD's for breakfast.
About 6:45 when I got there there were no fewer than 14 guys in the water, with another half dozen combing the sand. Some of them had head mounted flashlights do they could detect at 6am sharp. Managed to still find a couple of rings, but this illustrates the amount of competition at some places.
Places that allow detecting year round you won't see a swarm like that, but its easy to tell a regularly hammered beach - And if you detect enough, you'll meet some of these regulars. Some will be very friendly, some resentful that you are intruding on "their turf".
