The Isle of Sheppey is near and very popular, but I would avoid it. Too near the Thames estuary and I can't imagine the water is that clean. Further down is Whistable, but you have to check the tides, as when they are out, you have a long walk out to the sea across the mud flats, and I doubt if it is safe anyway.
The Thanet end has the advantage of sand instead of shingle, but I would vote for Camber as well, as it doesn't get so crowded. Or at least is didn't used to - it's several years since I visited it! Another advantage is that you can drive across to the Romney, Hythe and Dymchurch Railway.