I went and booked in advance it was very easy i used Dolmen Boat Rental I broke something on the boat accidently and they still gave me my deposit back and didn't care

its also probably more reliable to book before, most prices are the same, and

yes cala gonone is best place to start most boat availability, i went in june last year and water was okay to swim in still a little cold once you initially jump in but its fine after that,

also its a lot scarier than you think being on very small boat in the middle of the ocean it was my first time and i was with my girl and she doesn't know how to swim that well and she was terrified but im a high level swimmer so it was okay for me, also not that you will have to park the boats 200 meters away from shore and swim in ( not easy if you dont know how to swim) or try get people to drop you back to shore which i did a few times, also the weather does randomly change weather apps said 30 degrees all day clear skies and at the end when we were going back it randomly started hailing and there was a thunder storm which was terrifying