We did this back in 2001. We bought the tickets right at the dock and it cost $40 each, plus an additional $12 each for horseback riding at Yelapa. We took a boat called the Princess Yelapa. The tour included continental breakfast (which we didn't need as we ate on the ship), snorkeling and equipment at Los Arcos, a couple of hours at Yelapa to ride the horses, swim, etc., lunch on the way back, drinks (I think tequilla shots) and music/games/dancing.
It was a great day. I must caution, however. I do not know how many tours go out each day. We got off the ship, went to one of the little "travel agencies" to inquire about a tour. They offered us several trips and we chose the Yelapa tour. We gave this guy $160 and he ran over to a big line, cut in and bought us our tickets (at this point we're thinking "did we really just give this guy $160!?). Anyway, he did return with our tickets and told us to "run" because the boat was leaving NOW! We got on the boat, passing the tons of people waiting for a catamarn tour from the ship. After that, the day was very relaxing and stress free. We returned just in time for dinner (we had early seating - 5:45). As we pulled in, we could see our waiter having a smoke on the deck.....he spotted us and looked at his watch!
Our ship was in port from 8am-10pm so we did have a lot of time, just in case something went wrong.
So, long story short.......We loved this tour! Just try to find out information before you go about tour operators and length of tour, etc. so you can decide if it will be something you can take advantage of.