I've done tons of fresh water, lake, metal detecting. I did it in the 80s with a Garrett 500XL. I found lots of stuff. However, you have to think where the highest concentration of people are when they are at a swimming beach. It's actually not a lot deeper than waist deep. Unless there's a platform anchored in deeper water, sometimes with a diving board on it, I found no reason to go that deep. Now where the people swam out to the platform and rested and then dove back in to go ashore, you should find lots under that platform. My experience was I found it most productive to work between shoulder deep water and in as shallow as I could go. Sometimes I took the SCUBA off and snorkeled and crawled on my knees in 18" of water. The real advantage to diving is that you can see what you are after and then see it when you fan the dirt and sand away from the target. I stopped doing it after about 10 years as lots of others caught on to the trick and I found the 'regular' places hit pretty hard.My wife found a 14K gold ring with a diamond in it in 6" of water. A mom was probably sitting their in the water with her toddler swishing her hands in the water, and kerplunk. It slipped off. Wifey found it. Jim