Kamikat, Welcome. I cruised to Cabo in February. I also did the snorkeling excursion to Chileno Bay. Leave your things on the excursion boat. No one will bother your stuff. Everyone on the excursion is doing the same exact thing as you. The guides stay on the boat and very often, other passengers do as well. If someone is rifling through your belongings, someone will take notice of it. If you feel more comfortable, use a drawstring or zippered bag for the excursion and put your cash at the very bottom of the bag. I wouldn't waste anymore time worrying about it. Nothing will happen.
Just so you know... Chileno Bay snorkeling is just so-so. You are not allowed anywhere, and I do mean ANYWHERE, near the reefs. A man in a dingy with a smelly, smokey motor patrols the reefs and yells at snorkelers if they get too close. The beach is beautiful and you will see some some tropical fish. The water--very warm with good visability. Expect to snorkel in approx. 20-25 feet of water, maximum depth.
As others stated... Cabo will gladly accept U.S. dollars.
If you want to P.M. me with other questions, feel free.
Have fun and don't worry so much.