We booked a tour with Jet 2 which also included a bbq and stop off for swimming, not all the boats have tour guides, there are boats available at Elounda harbour for 10€ each which drop you at the island and return every 30... More
We booked a tour with Jet 2 which also included a bbq and stop off for swimming, not all the boats have tour guides, there are boats available at Elounda harbour for 10€ each which drop you at the island and return every 30 mins or so but these do not include a guide you also have to pay 8€ entrance fee on the island, we also went ourselves after the guided tour as they only walk around the outside and you don’t really get the chance to wonder off on your own to explore.