Santorini is one of the best places to go scuba diving because the island was created from a volcano many years ago which makes the sea life very different from other areas of Greece. There are very unique Coral reef Sandrock formations that have come from the volcano, and as always, the sea is teeming with wildlife. I highly recommend visiting Caldera Lagoon and arranging that you go scuba diving there. There are a few local schools that you should call ahead of time to schedule your slot. Best of luck!