There are 3-4 chances to go swimming in the Aegean Sea during the tour but the beaches are not sandy, they are made of small rounded rocks that hurt your feet. I would buy a pair of low cut divers booties and wear them swimming, I did this and it worked great. The booties mash down and pack well. The water is clear and beautiful, especially at Hydra. Trust me on this, it will make a huge difference in enjoying your time at the beach!