Underwater
There are outstanding locations for snorkeling or diving. Some are even close to the shore and not a boat necessary. Share special places you know and describe what to expect.
You get to the island only by ferry from Palma Rubia in the morning. In 2014 it was at 10 a.m.
Best during the dry season roughly from November to April but this is also the peak season. Shoulder season May and June is worth a try, less crowded but showers may occur
Cuba 
Snorkel and weather conditions at the Keys
The summer offers the flattest and clearest water for snorkelers. The downside the summer is also known for its uncomfortably high heat and humidity. In September and October, hurricanes can occur like in 2017. Much got destroyed by Irma here on the keys.
The months from November thru April are much more pleasant, but there can be strong wind and waves which reduce visibility substantially. Cold fronts from Canada with strong winds arrive here in the winter. The temperature drops, and it gets chilly. Also, there can be days where the surf is too high for snorkeling. The worst month with strong winds is January, and it improves in March. The best month for snorkeling is May until Memorial Day. It is still not too hot, water is clearer, and accommodations are not as expensive as in the summertime. September can be a good month as well but consider it’s the hurricane season.
Tip
Book your snorkel trip in advance and on a windy day give them a call in the morning to get the tour confirmed: (305) 451 6300
Avoiding crowds
If possible stay at the keys from Monday to Friday and avoid weekends. Weekends are utterly busy, prices for hotels increase and most spots are crowded. The advantage; on weekends guided trips are offered in the John Pennekamp State Park in the morning.
Camping in the John Pennekamp State Park
The popular campground is mainly used by RV’s each of the 45 sites has electricity. The campground provides washing machines and tumblers. If you like to stay here, you have to book 11 months before sharp at 8 in the morning. The alternative; usually sites get canceled shortly before the stay. Check it on Reserve America daily soon before you plan to visit the Keys.
We were forced to look for a motel. There wasn't any affordable campsite available. We stayed at the Conch On Inn for three nights. Luckily, a very comfy bed, WIFI, fridge, coffee machine, BBQ and attentive owners. Great value for money.
Hotels near the John Pennekamp State Park in Key Largo.
United States 
It is less busy in the morning and afternoon. We spotted more fish in the morning. Avoid midday too many snorkelers arriving by boat.
Climate and Crowds
Best months are during the dry season from December until May. Driest months are February to May. Day temperature always around 30 C°/ 86°F. Most sunshine in March. The rainy season lasts from June to November with the possibilities of hurricanes and flash floods. Wettest month is September. Minimum water temperature is 27 C°/ 81°F It's more crowded during the dry season, expect higher rates and crowds from mid-December to March. April and May rates decrease and "fewer" tourists except for Easter.
Saint Lucia