Are you talking about the municipal beach next to the pier and arcade? Don't know the name of that, maybe it's Geneva Beach or something like that.
Bigfoot is the small strip of sand across from the picnic area SE of the town, noted by the sign that says Bigfoot Beach State Park. A good long walk from the downtown area.
Fontana is on the west end of the lake just sw of Williams Bay.
In terms of volume finds, Bigfoot would be the one to hit, second to that would be Fontana as it's far more populated by locals who are far more casual on it. The downtown beach has as many tourists as locals. Never seen any experts on beaching talk about it, but I think the locally-used beaches may produce better than the tourist beaches.
However the population density and demographics point to Bigfoot as the best bet. I was sitting at the water's edge there one day taking a break and this 3 or 4 year old picks up a very nice ear stud that had to be a half carat and his granny snatched it up quick and threw it in her purse. All the while I was watching kids from tiny to teen dancing and playing rough in the water, gold rings and bracelets and chains adorned many of them generously, too boot.