Welcome aboard, Justin! Congratulation on planning your first sailing with Disney Cruise Line! Setting sail next spring on the Disney Dream will be such a magical vacation for the two of you!
The Bahamas boast fabulous weather year-round, so I'm sure you'll enjoy a nice visit to both Nassau and
Castaway Cay next May. The temperatures in May tend to be in the mid 70's to high 80's, so it's summer weather there! The water will definitely be warm and perfect for swimming. Even when we cruise in the fall and winter, we've experienced warm waters at the beach on Castaway Cay. It does tend to rain more often in that area, but normally, the showers are quick and light. It has rained a few times when we've been on Castaway Cay, but within twenty minutes to an hour, it passes and the sun has come back out to let us enjoy the rest of our day. More often, though, it has been simply beautiful all day long. The weather should be nothing to worry about for your trip, Justin, especially in May.
If you have any other questions about your cruise, feel free to stop back again! Happy planning!