On the Water · Diving
Diving safety in Puerto Rico
Who to call, how long to wait before you fly, the jellyfish days after each full moon, and how warm the water is month by month.
In an emergency
Call 911, then DAN. 787daily has not confirmed a direct Coast Guard number to print here: call 911.
- 911Call 911 first for anything life-threatening.
- +1-919-684-9111DAN emergency hotline (English; collect calls accepted)
- +52-557-100-0540DAN emergency hotline in Spanish
- +1-919-684-2948DAN non-emergency medical line (option 4; Mon to Fri, 8:30 to 5 ET)
- Coast Guard Sector San Juan runs search and rescue at sea, and flies maritime medevacs to Centro Médico (U.S. Coast Guard news release)
Centro Médico (ASEM), San Juan: hyperbaric medicine
UnconfirmedReported in 2020 as Puerto Rico's only 24-hour emergency treatment chamber, a steel multiplace unit for up to 20 people. Its phone and 24-hour status are not confirmed: call to confirm.
- "Puerto Rico's only 24-hour emergency treatment chamber" (2020) (Global Press Journal)
- ASEM's services page lists Medicina Hiperbárica, with no hours, phone or mention of diving (ASEM)
DAN's numbers from dan.org/contact (no date; checked Oct 18, 2026). DAN says to call local emergency services first in anything life-threatening.
Flying after diving
| Wait at least | After |
|---|---|
| 12 h | After a single no-decompression dive, wait at least 12 hours before flying. |
| 18 h | After multiple dives a day, or several days of diving, wait at least 18 hours. |
| 18+ h | After decompression, heliox or trimix dives, wait "substantially longer than 18 hours". |
- San Juan's airport is at sea level, so the rule applies to the flight itself. The guideline comes from DAN's 2002 and 2004 workshops (Divers Alert Network)
- Driving up the mountains counts too: Culebra Divers advises about 12 hours before driving to El Yunque's summit (4,390 ft, 1,340 m) after diving (Culebra Divers FAQ)
Culebra and Vieques flights are short hops in small aircraft; no source gave their cruising altitude.
Box jellyfish days
Box jellyfish (Alatina alata) come into shallow water and onto beaches about 8 to 10 days after each full moon. Avoid night swims and dives in that window, and wear a rash guard or a suit.
| Full moon (about) | Jellyfish likely (about) |
|---|---|
| Oct 25 | Nov 2 to Nov 4 |
| Nov 24 | Dec 2 to Dec 4 |
| Dec 24 | Jan 1, 2027 to Jan 3, 2027 |
| Jan 22, 2027 | Jan 30, 2027 to Feb 1, 2027 |
| Feb 21, 2027 | Mar 1, 2027 to Mar 3, 2027 |
| Mar 22, 2027 | Mar 30, 2027 to Apr 1, 2027 |
Moon dates counted by 787daily from the mean lunar month when this page was built (Oct 18), good to about a day.
- Box jellyfish arrive about 8 to 10 days after the full moon (University of Hawaiʻi News)
- Documented in Puerto Rico (Smithsonian research repository)
- A sting was reported at Crash Boat in September 2014 (JellyWatch sting report)
The water, month by month
Show the numbers
| Station | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Culebra | 80.9°F | 80.2°F | 80.8°F | 81.7°F | 83.2°F | 84.6°F | 84.9°F | 85.5°F | 86°F | 85.7°F | 84.1°F | 82.2°F |
| Esperanza, Vieques | 79.9°F | 79.7°F | 80.3°F | 81.1°F | 82.7°F | 84.2°F | 84.5°F | 85.1°F | 85.6°F | 85.2°F | 83.4°F | 81.2°F |
| Magueyes Island (La Parguera) | 80.7°F | 80.5°F | 81.4°F | 82.1°F | 81.4°F | 82.8°F | 83.9°F | 85.6°F | 86.1°F | 85.6°F | 83.9°F | 81.9°F |
| Mayagüez | 81.1°F | 80.8°F | 81°F | 82.2°F | 83.8°F | 85.3°F | 85.7°F | 86°F | 86.8°F | 86.5°F | 84.9°F | 82.5°F |
| Mona Island | 79.4°F | 78.9°F | 79.3°F | 80.5°F | 82.1°F | 83.4°F | 83.7°F | 84.7°F | 84.7°F | 84.1°F | 82.7°F | 81°F |
| Buoy 41043, 170 nm NNE of San Juan (offshore) | 79.6°F | 78.8°F | 78.8°F | 79.2°F | 80.8°F | 82.6°F | 83.6°F | 84.3°F | 84.8°F | 84.4°F | 82.9°F | 81.2°F |
| East Caribbean buoy, 180 nm SSW of Ponce (offshore) | 81.2°F | 80.4°F | 80.3°F | 81.2°F | 82.2°F | 82.9°F | 83.5°F | 84.4°F | 85.4°F | 84.9°F | 84°F | 82.6°F |
| Month | Water | Suit |
|---|---|---|
| Jan | 79.4–81.1°F26.3–27.3°C | 3–5 mm |
| Feb | 78.9–80.8°F26.1–27.1°C | 3–5 mm |
| Mar | 79.3–81.4°F26.3–27.4°C | 3–5 mm |
| Apr | 80.5–82.2°F26.9–27.9°C | Shorty or skin |
| May | 81.4–83.8°F27.4–28.8°C | Shorty or skin |
| Jun | 82.8–85.3°F28.2–29.6°C | Shorty or skin |
| Jul | 83.7–85.7°F28.7–29.8°C | Shorty or skin |
| Aug | 84.7–86°F29.3–30°C | Shorty or skin |
| Sep | 84.7–86.8°F29.3–30.4°C | Shorty or skin |
| Oct | 84.1–86.5°F28.9–30.3°C | Shorty or skin |
| Nov | 82.7–84.9°F28.2–29.4°C | Shorty or skin |
| Dec | 81–82.5°F27.2–28.1°C | 3–5 mm |
No San Juan station appears in NOAA's table. The latest readings on the page were dated 2026-08-21; the averaging period is not stated. The suit is Island Scuba's advice; the cool months are Rincón Diving's, December to March, as Island Scuba does not name its winter months.
- 3 to 5 mm in winter (Island Scuba)
- About 78°F December to March in Rincón: "most divers get cold in the winter months" (Rincón Diving & Snorkeling FAQ)
- A shorty or a skin in summer (Island Scuba)
- A 3 mm shorty year-round (the suit it lends) (Culebra Divers)
- December to April: clearest visibility; May to November: warmer water, often 84°F (Aquatica Underwater Adventures)
- Dry season, December to April: the best visibility and calm seas; hurricane season June to November (SSI blog)
- Mornings are calmer than afternoons (Rincón Diving & Snorkeling FAQ)
Hazards
- Lionfish
- Invasive. Take them freely outside reserves; inside reserves, or with scuba and a spear, only with DRNA's written lionfish authorization (see the reserves page). (DRNA Fishing Regulation)
- Long-spined urchins
- The 2022 die-off cut their numbers at several Puerto Rico sites by about 60% against 2012 and 2017, but they still make shore entries hazardous. (PeerJ)
- Fire coral
- Listed among the regulated coral families in Reg. 7949's appendix. No Puerto Rico sting data was found. (DRNA Fishing Regulation)
- Unexploded ordnance
- On Desecheo (the island is closed) and in the waters off the former Navy ranges on Vieques. (U.S. GAO)
- Rip currents and north swell
- Winter north swells bring High Surf Advisories and high rip-current risk to north-facing beaches, with seas of 6 to 10 ft and some events to 11 ft. NWS San Juan publishes an experimental beach and rip-current forecast. (St. Thomas Source)
- Reef-safe sunscreen
UnconfirmedNo Puerto Rico sunscreen ban was found in force. A bill to ban the sale of oxybenzone and octinoxate sunscreens (PC 645) passed the House 44 to 0 in February 2023; no Senate vote or signature was found. Operators ask for reef-safe sunscreen anyway.
- PC 645 passed the House in February 2023 (El Calce)
- An earlier bill, PC 1997, passed the House in November 2019 (EyBoricua)
- The U.S. Virgin Islands banned oxybenzone, octocrylene and octinoxate from 30 March 2020 (Stream2Sea)
Lionfish, spearfishing and the reserves' rules: reserves and rules. The NWS's beach and rip-current forecast.