Miami Beach in general is very relaxed and cool.
All of the suggestions made already are good ones. I'd also add 3rd Street Beach @Ocean Drive as its popular with Brazilians and gay alike.
I stay at the Delano Hotel, and thong at the pool there, and beach area in front. You can use the pool if you're not a guest
https://www.miamicurated.com/entertainment/miami-hotels-great-pools-offering-day-passes/
But the best SOBE area, as mentioned - 12th Street @Ocean Drive is just a short walk.
Also have had fun at Haulover (about 25 minutes drive from South Beach) but I find micro-suits sexier than nude. There's a mix of the two.
Have fun!