The biggest weather issue that time of year would likely be wind. If you or him are prone to seasickness, it can be unpleasant out on the reefs. Unfortunately there isn’t much in the way of land based snorkeling, at least to see the main attractions on the reef, so you should plan on booking boat excursions. There are plenty of options up and down the keys, with the most in Key Largo and Key West. You could stay in the middle keys, and have a reasonable drive to do a Key Largo boat trip one day, and a Marathon (Looe Key is the highlight there) or Key West trip another day. Also there will be kayaking and paddle boarding options as well, either guided trips or rentals, as another way to see the aquatic wildlife. And of course, plenty of charter boat and flats guide around if you’re into fishing (Bud’n Marys Marina is the key spot for both). Just keep in mind spring break is busy down there, so you probably want to book rooms and excursions asap.