Carlos, the guy at the sales stand at Marina (when you cross that bridge already) is very clear on explanation. Not pushy, not hard sales or anything.
I went on the €25 tour, bigger boat. This tour allows some times for you to swim/take photos. We'd cruise on big boat first, then at some point in the ocean, they'll split people into a group. (About 9 persons /group). Group will change to smaller boat, then went into those caves. So, when you're done with the small boat cave cruise/when you're waiting to go on a small boat cruise, you can enjoy the swim/relax/take photos.
No confusion for the meeting point. You can just ask people around the pier.
The tour guide was professional, safe & English-speaking. No problem at all.
Normally, you can show up, book/pay, and just go. But sometimes, internet bookings can come in and makes them full. So, I'd recommend if your schedule is not flexible/you want sureness, maybe better to book 1-day in advance at a Sales stand.
Overall, great experience :)