Nitrox Diving at Cocos Island: A Practical Guide

Cocos Island is Costa Rica’s most remote national park and one of the world’s great shark dives, and the only way to dive it is from a liveaboard boat that spends a day and a half crossing the open Pacific to get there. Most dives at Cocos are deep, most divers want to stay down with the hammerheads as long as possible, and the nearest recompression chamber is a long sea voyage away. That combination is why enriched air nitrox has become standard equipment on Cocos trips. This guide explains what nitrox does and doesn’t do, how a Cocos trip works, and where to stay on land before and after.

Cocos Island in brief

Isla del Coco sits far out in the Pacific. The Cocos Marine Conservation Area, the branch of Costa Rica’s national park service (SINAC) that manages it, places the island about 496 kilometers southwest of Cabo Blanco on the Nicoya Peninsula. The island covers about 24 square kilometers. It was declared a national park on 22 June 1978, listed by UNESCO as a World Heritage Site in 1997, made a Ramsar wetland site in 1998, and named part of Costa Rica’s historical heritage in 2002. Its tallest peak, Cerro Iglesias, rises to about 634 meters.

The protected waters around it are now far larger than the original 12-nautical-mile ring. In December 2021, Executive Decree 43368-MINAE expanded the national park from 2,034 to 54,844 square kilometers and enlarged the surrounding Bicentennial Marine Management Area to about 106,285 square kilometers, as reported by Delfino and by Sylvia Earle’s ocean conservation group Mission Blue. Fishing is prohibited inside the park, and controlling illegal fishing is the main job of its ranger patrols.

The draw underwater is big animals. Undersea Hunter, one of the island’s long-standing liveaboard operators, lists 12 shark species, four kinds of rays, four sea turtle species and at least 27 endemic fish, including the rosy-lipped batfish. Schooling scalloped hammerheads are the headline, with whitetip reef sharks, Galápagos and tiger sharks, marble rays, mantas and the occasional whale shark also on the list. None of these is guaranteed on any given dive.

Background on the island’s history and logistics is also available from Costa Rica Journeys on Cocos Island.

How a Cocos dive trip works

Nobody stays overnight on Cocos except park rangers and researchers; there’s no lodging and no camping. Entry is by permit from the conservation area under the park’s public use rules, and in practice divers arrive on licensed liveaboards that handle those permits. The two operators most divers will find are the Aggressor fleet, which runs the Okeanos Aggressor II and the Cocos Island Aggressor, and Undersea Hunter, which currently sails the 115-foot Sea Hunter while its larger ship, the Argo, is away on a National Geographic Pristine Seas expedition.

The basic pattern is similar across boats:

• Departure: from Puntarenas on the Gulf of Nicoya, usually after a bus transfer of about two and a half hours from San José hotels.
• Crossing: 32 to 36 hours each way across open ocean. Seasickness remedies are worth packing.
• Length: Aggressor runs 8, 10 and 12 night charters; Undersea Hunter runs 11 and 14 day expeditions.
• Diving: usually three dives a day from small tenders or skiffs. Aggressor quotes typical depths of 16 to 33 meters (55 to 110 feet) and notes that night diving isn’t currently offered at Cocos.
• Shore visits: usually one or two landings on the island during a trip.

Cocos isn’t a beginner’s destination. Undersea Hunter’s FAQ says the minimum certification is Open Water but strongly recommends Deep Diver training and at least 25 hours of logged diving, because most dives go past 18 meters and currents and visibility can change within hours. It also requires guests to carry Divers Alert Network (DAN) insurance, and it caps dives at 30 meters and 60 minutes with no decompression diving allowed, since the nearest hyperbaric chamber in San José is about 36 hours away.

What nitrox actually does

Enriched air nitrox is simply breathing gas with more oxygen and less nitrogen than ordinary air, which is about 21 percent oxygen. The common recreational blends are 32 and 36 percent oxygen, written EAN32 and EAN36.

The benefit is about nitrogen loading and no-decompression limits. Every dive pushes nitrogen into the body’s tissues, and a dive computer tracks how close a diver is to the point where a direct ascent would carry a risk of decompression sickness. Because nitrox contains less nitrogen, tissues load more slowly, so the computer allows more time at the same depth before a mandatory decompression stop. PADI sums it up as longer dives and shorter surface intervals. At Cocos, where dives sit in the 20 to 30 meter range three times a day for a week, that extra margin is noticeable.

It’s worth clearing up a common mix-up. Nitrox is often sold as a fix for nitrogen narcosis, the drunk-like impairment of judgment that sets in on deeper dives. The evidence doesn’t support that. A 2015 review in Extreme Physiology and Medicine notes that breathing compressed air deeper than about 30 meters invariably produces narcosis, that effects can appear at much shallower depths, and that oxygen itself is ranked as more narcotic than nitrogen. The simplest remedy remains ascending a few meters. Nitrox is a decompression tool, not a narcosis cure.

The trade-off: oxygen limits

More oxygen brings its own limit. Breathed at high pressure, oxygen can cause seizures, and a seizure underwater can easily lead to drowning. DAN notes that the old working limit of 1.6 atmospheres of oxygen partial pressure has largely given way to 1.4 for the working part of a dive, and urges divers to respect the maximum operating depth (MOD) of each blend. The richer the mix, the shallower that limit.

These depths are calculated from the 1.4 limit; the boat’s dive masters and each diver’s computer set the actual plan. Because Cocos operators already cap depth at around 30 meters, EAN32 fits the island’s profiles well, which is why it’s the usual fill. Every nitrox diver analyzes their own tank before the dive and sets their computer to the measured mix.

Getting certified

Boats only give nitrox to certified divers, so bring the card. Undersea Hunter says divers who aren’t yet certified can take the theory course during the crossing, a session of about three hours with no in-water skills required, and that nitrox fills are free and unlimited on board. Aggressor also lists nitrox for its Cocos boats. Taking the course before the trip, through any PADI, SSI or NAUI dive center, leaves the crossing free for rest and gear prep, and the online theory can be done at home. Divers who want deeper margins should also consider the Deep Diver specialty that Undersea Hunter recommends.

When to go

Liveaboards run to Cocos year round. Divers and operators broadly split the year into a drier season (roughly December to May), with calmer crossings and clearer water, and a wetter season (roughly June to November), when seas are rougher but many divers report larger hammerhead schools and better odds of whale sharks. Hammerheads are present in every month. Booking well ahead matters more than the month, since boat space is limited and trips fill early.

Water temperatures are generally 24 to 29°C (76 to 83°F) according to Undersea Hunter, with colder thermoclines at depth; a 5 mm full wetsuit is the usual choice.

Before and after: where to stay on land

The crossing schedule depends on tides, and returns can run late, so Undersea Hunter strongly recommends spending an extra night on land before flying home. That night, or a few days on either side, is a good chance to dive or snorkel somewhere very different.

Close to San José airport. Finca Rosa Blanca is an organic coffee farm inn in Santa Bárbara de Heredia, in the hills above the Central Valley, with 14 villas, suites and studios and guided farm walks. It’s a calm place to recover from the crossing before a flight. More options are in our Central Valley region guide.

Drake Bay and Caño Island. For a few days of reef diving with no long crossing, Caño Island Biological Reserve off the Osa Peninsula is one of the country’s best-known reef dives, reached by a short boat ride from Drake Bay. Copa de Arbol, a boat-access beach lodge of nine villas just south of Drake Bay, runs Caño Island scuba and snorkel trips. Aguila de Osa Inn on Drake Bay operates two-dive Caño Island trips with small groups and can finish Open Water certifications, which suits newer divers building experience before attempting Cocos. La Paloma Lodge, with 11 bungalows and rooms above the bay, arranges Caño Island snorkeling for non-divers in the group. Our Osa Peninsula region guide and Corcovado and Osa Peninsula guide cover the area in more depth.

For the wider picture, our guides to snorkeling and scuba diving in Costa Rica, Costa Rica’s marine life, whale watching and dolphin tours and Costa Rica’s national parks fill in the rest.