The best is south of the island at blackgang chine.
I was there 20 years ago, but I believe there has been coastal erosion since.
A quick net search should give you further details.
I found the local nudies very friendly & helpful.

On the South West coast Atherfield is easily accessible. Park on the grass verge, unless you are camping at the adjacent campsite, not open in the winter.
It's pebbles at the Chine but go left to the Point and beyond and it's firm sand. You can naked walk all the way to Blackgang along the beach but beware of the tide height at the Point and beyond.
Beware also of the fierce currents offshore, as you are in the English Channel!

Yaverland, North end of Sandown, is naturist at the far limit, below Culver Cliff. The wide sandy beach there fizzles out at the Cliff end. Parking and facilities are adjacent to the yacht club, but it can get full in the season.