Costa Rica
Playa Grande next to Montezuma. Loved the community there. The beach is unofficially CO, but widely known as CO. I used it as such but saw only one other couple taking advantage of it. They were going to rush to put there swimwear on until I pulled my trunks off. That said, it wasn't the lack of nudist on the beach, it was the lack of people. One day I was there, there was about six people total on the half mile long beach. So, I can't vouch for how CO the beach is when there are more people there. I'm sure its never very crowded. That's not the character of Montezuma. There are no big resorts nearby.
The beach just down the way from Montezuma is also called Playa Grande -- confusing. https://montezuma-costarica.com/montezuma-beaches/
Even on this link they suggest that topless sunbathing is okay on the beach. A lot of other sites say it is clothing optional, which seems to be pretty well accepted.