Welcome aboard, Julianne! Your magical Dream sailing isn't too far off, is it? April is just around the corner!
Atlantis is a must-do when you're in Nassau, so I'm glad you thinking of doing the water park there. The weather will be perfect for some wet and wild fun, I'm sure. Temperatures in April in Nassau tend to be in the upper 70's and low 80's. You'd be best to check the weather online about a week before you sail for a more accurate forecast.
Disney's Port Adventures can be cancelled up to three days before you set sail, so you still have some time to decide.
I will say, though, that if you are not set on doing this excursion, that day in Nassau is a great day to stay on the ship! During our last trip to Nassau on the Dream, we stayed on the ship and took advantage of less crowds at the pools and our favorite: little to no wait at all for the
AquaDuck! It's also a great day to wander the ship or sign up and begin the Midship Detective Agency which will take you all over the ship on multiple decks to solve one of two cases. You'll hold up a card to the magical artwork and watch it come to life, giving you clues to the mystery. It's fun for all ages!
Have fun planning your cruise, Julianne! April will be here before you know it! Let me know if I can help with anything else before you set sail!