We don't typically drink so I can't help you there. Your son will be able to swim in the ocean on
Castaway Cay. There are plenty of lounge chairs and umbrellas on the beach. You can rent tubes or floats on the island. There are always plenty so no need to do it ahead of time in my opinion. Be aware that sometimes that time of year, the weather isn't the greatest so Castaway Cay may be skipped due to high winds and not being able to dock. I'll assume your other stop is Nassau. Choose an excursion that looks like something you would like if you want to do an excursion. There are all price ranges for excursions. We don't get off the ship anymore in Nassau unless we are on an excursion. The shopping is good but we've had to put up with very pushy locals which I hate. Keep in mind that in their culture, you pick it up or you try it on (sometimes they will try to put it on you to get you to buy it), like a bracelet, you've basically bought it. Shop with your eyes!
For the first day, you will not have access to your luggage until the late afternoon. Anything you need, swimsuits, medication, etc. keep with you in your carry on. You can have lunch on the ship when you board. My kids then change and swim. Your cabin will be available about 1:30ish. I usually go and unpack while DH watches the kids swim. There will be open houses for the kid's clubs for your family to check out the spaces. My kids never want to leave, they have so much fun.
We enjoy dinner in the main dining rooms. On a 3 night, I'd for sure eat in the dining room each night. The shows are fun to see. You will get a Navigator each night outlining all the activities. Take a highlighter so you can mark your not to be missed activities. Also download the app so you can see the activities on your phone. You don't need to pay for the ships wifi to take advantage of this app or the texting to another person on the ship.
If you want to get to know some of your fellow cruises, you can find your cruise under "Meet and Greet" on this board. You may want to get involved in an FE exchange. However, don't get so overwhelmed that you don't enjoy the cruise. We've always enjoyed the FE exchange but some people find it more work than fun.
Have a great cruise!