Diving in Costa Rica (Cocos) in December

Conditions in December

So, you’re thinking about diving Cocos Island in December? Excellent choice! December marks the transition from the rainy season to the dry season, creating a fascinating blend of conditions. While you might still encounter some passing showers, these are generally short-lived and definitely won't dampen your underwater adventures. The sea state during this time is typically calm to moderate, offering comfortable transits to and from this remote paradise. You can expect fairly consistent surface conditions, making for smooth sailing on your liveaboard journey.

Water temperatures around Cocos Island in December are still on the warmer side, usually sitting comfortably between 26°C and 28°C (79°F to 82°F). However, be prepared for thermoclines! These can drop the temperature by a few degrees, especially on deeper dives or in areas with strong upwellings. So, while a 3mm wetsuit is generally sufficient, some divers might prefer a 5mm for extended dives or if they tend to get cold easily. Visibility is generally good, typically ranging from 15 to 25 meters, though it can occasionally be impacted by recent rainfall or plankton blooms – but these blooms are often great news for feeding pelagics! Currents can be moderate to strong, which is a hallmark of Cocos Island diving, and are often responsible for bringing in the nutrient-rich water that attracts the incredible marine life.

What you will see

December at Cocos Island is absolutely prime time for pelagic action, and that's precisely what Cocos is famous for! The schooling hammerheads, specifically scalloped hammerheads, are a consistent highlight, and in December, their numbers are fantastic. You’ll regularly encounter large schools swirling around cleaning stations, offering incredible photographic opportunities and a truly unforgettable experience. White-tip reef sharks are ubiquitous residents, often seen resting in crevices or cruising the reefs. You'll also likely spot Galapagos sharks, often in curious and bold encounters.

Beyond the sharks, December brings in other exciting visitors. Manta rays are frequently sighted, gracefully gliding through the blue, and eagle rays are also a common sight. Keep your eyes peeled for marbled rays and cow-nosed rays too. The deep blue brings with it the possibility of oceanic whitetip sharks, though these are less predictable. Whale sharks can also make an appearance, particularly when plankton levels are higher, though they are an incredible bonus rather than a daily guarantee. The rich currents are a magnet for jacks, snappers, and barracuda, often forming impressive schools that shimmer in the sunlight.

While Cocos is renowned for its big stuff, don't forget to appreciate the macro life that thrives on its healthy reefs. Though not the primary draw, you’ll find vibrant nudibranchs, colorful reef fish, and various crustaceans tucked away in the coral and rock formations. The sheer productivity of the ecosystem means there's always something to see, whether it's the smallest blenny or the largest pelagic.

Top dive sites this month

When diving Cocos Island in December, certain sites really shine due to the prevailing conditions and marine activity. Manuelita Coral Garden often delivers spectacular hammerhead encounters in December. The cleaning stations here are bustling, and you can expect to see large schools circling the pinnacles. The shallower coral garden also provides a beautiful backdrop for white-tip reef sharks and plenty of reef fish.

Another must-visit site is Dirty Rock. This iconic seamount is a magnet for pelagics, and in December, you’re practically guaranteed to see schooling hammerheads, Galapagos sharks, and often manta rays cruising by. The currents can be stronger here, but the rewards are immense. Don't miss Alcyone, perhaps the most famous hammerhead dive site in the world. The deep pinnacle provides unparalleled opportunities to witness hundreds of hammerheads ascending from the depths for cleaning. December's conditions often perfectly align for these breathtaking spectacles. For varied topography and marine life, sites like Submerged Rock and Chatham Bay also offer excellent diving, with chances to spot various sharks, schooling fish, and beautiful reef scenery.

Practical tips for your trip

Given the water temperatures and potential thermoclines in December, a 3mm full wetsuit is a good baseline, but if you're prone to getting cold or plan on multiple dives a day, a 5mm or even a 3mm with a vest would be a wise choice. Remember, you’ll be doing multiple dives daily from your liveaboard, so staying warm is key to enjoying every minute. This month generally suits experienced divers best due to the strong currents that are often present, especially at sites like Dirty Rock and Alcyone. While some sites offer more sheltered options, much of the best action is where the currents are strongest. Beginners should have a good level of comfort and experience with drift diving before embarking on a Cocos trip.

Liveaboard availability for Cocos Island in December can be competitive, as it’s a popular time to visit. It’s highly recommended to book your trip well in advance, usually 6-12 months out, to secure your preferred vessel and dates. Besides your standard dive gear, consider bringing a reef hook (used responsibly and only on dead rock!) for observing hammerheads in strong currents. A good quality underwater camera is also a must-have for capturing the incredible pelagic action. Make sure you have appropriate travel insurance that covers diving in remote locations, and don't forget essentials like seasickness medication for the long journey to the island, sunscreen, a hat, and insect repellent for any topside excursions. Keep in mind that Cocos Island is a national park, so be respectful of the environment and follow all park regulations.