Santorini is known more for its cliffs than its beaches. And it does have some beautiful beaches.
The Red beach is near Akrotiri so we visited while we were there. We didn’t actually go down to the beach; we figured the view from the distance was the main thing and didn’t feel a need to go down to the beach itself.
Below is Perissa beach. We stayed near here. It’s black sand, which is of course super hot. There are lots of boardwalks to get you to loungers and umbrellas without burning your feet but unless you’re good at sprinting in sand I recommend some kind of footwear! The beach is beautiful with the dramatic headland at one end, and it’s quite long. While a lot of it is organized beach there’s still at least one area of empty sand.
Here’s a sign for one of the many beach restaurants.