Two stops at Castaway Cay!? Wow - that will be great! Guessing you've been to Florida a few times and know that if/when it rains during part of the day, it doesn't rain all of the day (except for a few of our late August "Hurricane" trips).
If you don't get off at Nassau, you won't miss much. Maybe you and others won't spend your entire day at Castaway Cay, but let's put it this way . . . If you go snorkeling, the fish won't mind that it's raining.
How many $$$/#'s of non-cancellable excursions are you down for?