Ahoy there, Nancy, and thanks for floating by planDisney.
I can't wait for you to experience the 
Wish! She is such a beautiful ship, with so many magical things to experience onboard. When we watched Disney's Aladdin - A Musical Spectacular, my entire family agreed it was as good as 
Aladdin on Broadway!
Currently, Disney does not offer 
religious services onboard, and 
Castaway Cay does not have a chapel on the island, as it is Disney's private island. But! You will be in Nassau on Saturday, and I would see if any of the churches or cathedrals on the island offer Mass. The 
ship is in port from 930 am to 530 pm, so if there is Saturday Mass on the island, you should have plenty of time to attend.
Nancy, when you embark on the ship, stop by Guest Services and see if the Cast Members have additional information about celebrating the sacrament in Nassau. They will be happy to help if they have any specific information to share.
For up-to-date information while you're at sea, download the 
Navigator app. It will keep you abreast of dining, entertainment, character greetings, and more while onboard. My favorite part of the app is the ability to "heart" activities. When you do this (and allow notifications on your smartphone), the app will alert you ahead of your activity, giving you plenty of time to ensure you're not late for your very important date!
If you have any other questions, please don't hesitate to sail on back. Bon Voyage!
Rayna