In Santorini, if you are only visiting Fira and/or Oia you will have an amazing view but you will be on top of an impressive cliff and consequently, no access to a beach. The caldera is on the West coast and the beaches are mostly on the east coast.
However, if you plan to visit more than Fira and Oia, you can swim from a black, red or white sand beach on the main island of Santorini.
For the Red Beach, you can combine with a visit of the Akrotiri archeological site and the lighthouse. Parking is free at Red beach but limited. You will be surprise by the nice natural art in the rock. Those beautiful depressions in the rocks made by the ocean, the wind and time gives you a beautiful background. The access is tricky and not accessible for a wheelchair. You have to climb over boulders to reach the beach.
The Black beach of Perissa is beautiful with restaurants nearby. It connects with other beach and when you are in this area you have a bare white rock mountain, home to the ancient Thera, Mesa Vouno, in the background.
The natural surrounding is definitely a nice feature when on the beaches of Santorini.