Dave explained this perfectly. Carnival's policy is to give a refund if the excursion is canceled.
If you have pre-booked online and the excursion is canceled on board, they will issue a credit on your Sail & Sign account. Any amount over your Sail & Sign balance will be given as a check at the end of the cruise.
They can be a "little" flexible about the refunds if the weather is "iffy" so to speak the early morning of the excursion while waiting for confirmation from the tour operator if you ask nicely at the Information Desk. They have refunded on board charged excursions in the past for me. It is the exception however and not the rule.
Most excursions are "rain or shine' without a Guarantee of weather conditions.