You can take your own snorkel gear, but yes, anyone who chooses to go out and snorkel must have a neon green life preserver on at CC - you can get one for free from any of the lifeguards.
NOt only am I a TA, but I just got off the
DCL 7 day - so it's still fresh in my mind how many time the lifeguards would yell that through their bull horns to people without the life preservers on. But as for ages of snorkelers, can't remember that being yelled. If your son is comfortable with snorkeling already, I think no one will care or card him if he is with you guys.
The 3 day is more casual cruise, there is no formal night. The dressier night is slacks and nice shirt for men and dress/capris/slacks and nice shirt for women or variations on that theme.
Rule of thumb: no shorts, no tank tops (men), no bathing wear into the dining rooms.
I am in the camp of wear something nicer, like you would to a nice restaurant at home, to dinner - but it doesn't concern me for the 3-day as long as people aren't showing up with the above mentioned items - plus I learned how shockingly isolated you are at your tables. They have the dining rooms set up very nicely, in such a way that you are only aware of those tables right around you and not the entire dining room like on other lines.
I am going to see how little I can pack for this cruise. If I can stick to carry on items, I will be thrilled. I think I can pull it off. So I am going to be doing some creative planning for dinner wear.
Sneaking in for a minute from work just to let ya'll know I am still "on page 1" 
and to say "Hi".
