We will be on a cruise in August and will tender in Cannes. We would like to check out Nice and spend the afternoon at one of the private beaches. Does anyone have recommendations? I would really like one with a locker so we can swim without worrying about our things.
I would encourage you to bring the absolutely least possible in the way of valuables off the cruise ship. We ran into someone who lost thousands of dollars, their passports, their valuable cameras etc from a locked locker at the baths in Budapest. When you are not familiar with an operation, you can't know how secure such amenities are so have the money for the day and such but leave anything you don't need on the ship.
The Nice tourist office maybe able to help. Send an email.
Picking a private beach is a good idea, the coast generally has pebbles or very sharp gravelly sand. Only the private beaches have sand as you might expect sand to be. Expect to pay handsomely for the privilege.
As such, because I am cheap, I can't suggest one over the other.
You my find the beaches at Antibes more sandy, and closer to Cannes.
The Ambassador in Juan-les-Pins was absorbed by the AC Hotels chain but their private beach still retains an air of exclusivity and the beach is actually sandy rather than pebbly. Not as blue as in pebbly Nice, but more comfortable. You have to make reservations ahead, and they expect you to pay not just for the chair/towel/umbrella, but will also encourage you to drink and snack, and will have service staff giving you personal attention. Keep in mind that many go topless.