More than 80 if you hug the coast. The pub down in Niton is called the Buddle Inn. Great little pub. Good place to bivvy would be somewhere on the cliffs by Rocken End/Blackgang. There is even an abandoned old nudest camp hidden in the trees there on the cliff if you can find it.
If you start at Ryde though perhaps it would be best to Bivvy on the cliff/beach at compton (further west) but only in good weather, there is no shelter although there is a crapper in the beach car park though.
You can walk almost the whole length of the south west side from compton farm to st. catherines on the beach. Only at very extreme low tide can you make it from compton farm to freshwater bay.
If you do that, please take your little home, I don’t like like having to clean up after overners and its my bestest most favourite place in the world 🙂