Cave Tubing and any ruins "tours" require transportation to the mainland, thus the cost.
You could kayak out to the reef to snorkel, but I wouldn't, the only other way is with a for hire guide or one of the "tour" companies because there are no boat rentals. We did several snorkel outings with Tulu, his boat is Janel docked at the Courtasea dock just north of SP. Excellent guide, great guy. Also, used Amigo Del Mar, also good.
We plan to cave tube this year, I have heard that the "tour" thru Jaguar's Paw is excellent and it includes a visit to the Belize Zoo. Might want to check the cruise ship schedules before setting your day, the less people, the better the trip.
Good luck, and enjoy, it is a wonderful place.