Are you booking with the tour operator or thru Disney?
ANY excursions booked thru Disney are fully refunded if the excursion cannot happen for whatever reason, weather, missed port, etc.. I would also say you should always book the excursions early, I believe you're allowed 60 days out from the cruise. Excursions do sell out by the sail date.
It's up to the individual tour operators when you book direct, make sure you're very clear on it when you talk to them.