Castaway (1987)
That one's a fan favorite. And I'm a fan.
The way Amanda Donohoe casually just whips of all her clothes when they get left on the island is a "true nudist moment" rarely seen in movies. And it's true to the book too. As Lucy Irvine herself explained:
"On the first day on the beach, I just threw everything off. It was hot. There was nobody else there. Nobody was going to see what we got up to. And he was my husband, so that was fine, and I didnt really think very much of it. What was quite funny was that, as time went on, he began putting things back on because the mosquitoes and the sandflies get in all sorts of places you dont want them to be. It ceases to be erotic quite early on. You stop thinking about sex all the time with somebody whos naked when theyre naked all the time."
It's rare, especially nowadays, that an actress embraces the spirit of the author when it involves nudity. And Amanda Donohoe did it with such gusto that she deserves nothing but praises for it.
Not the Tom Hanks one. This one featured Amanda Donohoe is extremely long nude scenes through much of the movie. Best Oscar for no inhibitions in a clothing optional role!
If I recall correctly, while Cast Away (2000) had no full nudity, Tom Hanks used pieces of his clothing to meet various needs on the island and by the time he sailed off and was rescued, he had just enough left to avoid an R rating.