Sonora, Mexico · Sea of Cortez
A field guide to the reef fish, sea lions and big animals, small creatures and birds of San Carlos and Isla San Pedro Nolasco, with where and when to look for each and links to go deeper.
Photo Megan Applegate (Malpelo, Colombia)
San Carlos is a seaside town on the Sea of Cortez, within the municipality of Guaymas, Sonora (Wikipedia), about 20 km northwest of Guaymas and about two hours by road from Hermosillo (México Desconocido). Above the bay rises Cerro Tetakawi, about 200 m high, whose Yaqui name refers to its twin-peaked shape (México Desconocido). It became a Pueblo Mágico in 2023 (Visit Mexico), and the film Catch-22 was shot there (Wikipedia). US divers often drive down: San Carlos is about six hours from the border on Highway 15 (Wikipedia), and it lies inside the Sonora free zone, where no vehicle import permit is needed (BajaBound). The nearest airport is Guaymas (GYM) (AC-U-KWIK).
The headline dive is Isla San Pedro Nolasco, a steep island about 28 km offshore with a breeding colony of California sea lions (Wikipedia; IUCN World Heritage Outlook). The islands of the Gulf of California, Nolasco among them, are part of a World Heritage site inscribed in 2005, where IUCN rates overfishing and bycatch as very high threats (IUCN World Heritage Outlook). Closer to town are rocky reefs, islets with arches and the Ex-Santos, a navy ship sunk as an artificial reef in 2022 (Ikelite; Undercurrent); three dive shops run boats from San Carlos (Ikelite).
Location
San Carlos on the Sonora coast and Isla San Pedro Nolasco, the sea lion island, about 28 km offshore.
Dive sites, from west to east
The site map works offline and prints. Satellite is interactive: zoom and drag, and tap or hover a pin for its name (it needs an internet connection). Positions are approximate. Nolasco’s South Point, San Antonio, Eagle Rock, Tres Marías, the Sea Mount, the older wrecks and the other SAAS ships have no published position, so they have no pin. Positions: NGA GeoNames; the Ex-Santos from SEMAR. Coastline © OpenStreetMap contributors.
Four field pages, with where and when to look for each species and links to go deeper.
Pick a month to see what is at its peak. San Carlos is dived all year, but the water changes a lot: monthly averages at Guaymas run from about 17 °C in February to about 31 °C in August and September (seatemperatu.re). Wikipedia gives surface averages of 29–32 °C from June to August and 17–22 °C from November to March (Wikipedia). Visibility is often over 30 m and can exceed 60 m at San Pedro Nolasco (Wikipedia). The eastern Pacific hurricane season runs from 15 May to 30 November (NOAA).
In the Gulf, sea lion pups are born from late May, mostly in June, and the breeding season runs from June to August (Adame (IPN)); large numbers of pups can be seen at Nolasco in summer (Wikipedia). Bulls guard the rookeries then, so keep your distance (California Diver).
Monthly averages at Guaymas are 28–31 °C from June to October (seatemperatu.re). One dive magazine puts the peak dive season from late May through October (Scuba Diving).
Hammerheads are best from August to November, with schools at San Pedro Nolasco for experienced divers (PADI).
Mobula rays are seen in winter, and bullseye electric rays and banded guitarfish turn up now and then (PADI). The water is coolest then: about 17–19 °C on average from December to March (seatemperatu.re).
Background photo: Megan Applegate (bigeye trevally, Socorro, Mexico)
Six animals divers meet at San Carlos and San Pedro Nolasco, with the mark that gives each away. The species pages have the full notes and sources; the sea lion was photographed at San Carlos, the others elsewhere in the eastern Pacific.
From Wikipedia, dive magazines, dive shops and trip reports; some reports are several years old. Individual dives vary.
“Seal Island”: a steep island 4.2 km long and 328 m high, about 28 km west of San Carlos, 75–90 minutes by boat, with a sea lion colony and visibility that can exceed 60 m.
Source: Wikipedia; Undercurrent; WikipediaA swim-through down to about 23 m where hammerheads are often seen.
Source: Scuba DivingThe former Mexican Navy minesweeper ARM Santos (ex-USS Gladiator), 67 m long, sunk as an artificial reef on 3 March 2022 off Punta Cautín. It sits upright on sand in about 16–20 m (sources differ), with the top 3–5 m down; Tropical Storm Kay broke off the bow in September 2022. It was the first ship of the SAAS artificial reef (see below). Going inside needs Advanced Open Water certification.
Source: Ikelite; Gary’s Dive Shop; SEMAR; Dive San CarlosTwo islets, each with an underwater arch; divers have been shown seahorses there.
Source: UndercurrentA wall dropping to about 21 m, popular for night dives.
Source: Scuba DivingCurrent-swept boulders covered in sea fans.
Source: Scuba DivingThree peaks about 15 minutes east of town, with schooling hammerheads at about 34 m and occasional mantas and whale sharks.
Source: Scuba DivingA 2010 report describes the tuna boat Albatun (about 55 m, in about 21 m) and the 100 m ferry Presidente Díaz Ordaz north of town. Their condition today is not known.
Source: Scuba DivingBefore you go
The Mexican Navy suspended diving at the Ex-Santos for a time after the sinking (SEMAR), and the bow was broken off by a storm later in 2022 (Ikelite); ask a local dive shop about current conditions. The Ex-Santos is marked by a yellow buoy that must not be used for mooring (SEMAR).
Photos
Photos marked “Megan Applegate” were taken at San Carlos in July 2024 (the sea lion, the jewel moray and the sea slugs) or show the same species at Malpelo, Cocos Island or Socorro, labelled with the place; the others are credited reference photos, and species without a photo show “Photo to come” or link to photos online.
Background photo: Megan Applegate (snappers, Malpelo, Colombia)
Retired Mexican Navy ships sunk off San Carlos as new habitat and as wreck dives.
The Sistema Arrecifal Artificial Sonorense (SAAS, the Sonoran Artificial Reef System) is a project led by the Mexican Navy (SEMAR) with the government of Sonora, near Cerro Tetakawi in San Carlos (Vanguardia). It began in March 2022 with the sinking of the former minesweeper Santos (Vanguardia). By February 2026 it held four ships plus a helicopter, an airplane fuselage, an amphibious vehicle and three naval guns, ten structures of fifteen planned, and was described as more than 70% complete (Vanguardia). An underwater museum made up of six of the sunken naval artifacts, including the airplane and the helicopter, was inaugurated on 1 February 2026, the day the patrol vessel Cabo Corrientes was sunk with President Claudia Sheinbaum and Governor Alfonso Durazo present (Vanguardia; El Imparcial). The patrol vessel Mariscal followed on 11 July 2026 (La Crónica de Hoy).
The reef is already full of life: about nine thousand fish and at least ten invertebrate species have been identified there, among them sardines, oysters, grunts, octopus and snapper (Vanguardia; El Imparcial). A local dive shop reports morays, octopus, crustaceans, nudibranchs and occasional rays on the wrecks (Dive San Carlos).
| Ship | Type | Sunk | Length | Depth | Sources |
|---|---|---|---|---|---|
| Santos (PO-106) | Minesweeper, formerly USS Gladiator | 3 March 2022 | 67 m (221 ft); “over 70 m” per Dive San Carlos | About 16 m (52 ft); 20 m (65 ft) per Gary’s | Ikelite; Gary’s Dive Shop; Dive San Carlos |
| Suchiate (BI-05) | Navy vessel | 2024 | About 55 m (180 ft) | 15–17 m (50–55 ft), upright on sand | Dive San Carlos |
| Eltanin | Small patrol vessel | Early 2025 | Not reported | About 13 m (42 ft) | Dive San Carlos |
| De la Fuente (PC-208) | Patrol vessel | April 2025 | 34 m (110 ft) | About 16 m (52 ft) | Dive San Carlos |
| Cabo Corrientes (PC-271) | Coastal patrol vessel | 1 February 2026 (one report says 3 March) | Not reported | Not reported | Vanguardia; El Imparcial; La Crónica de Hoy |
| Mariscal (PC-211) | Patrol vessel, built in Troon, Scotland; in service 1975–2021 | 11 July 2026 | Not reported | Not reported | La Crónica de Hoy |
Diving the wrecks
The wrecks lie at about 12–18 m (40–60 ft), within reach of recreational divers, and several are close enough together for two or three wrecks in a day; they are reached by boat from San Carlos, and the tops of the Santos and the Eltanin can be seen by snorkellers (Dive San Carlos). Going inside the Santos needs Advanced Open Water certification (Gary’s Dive Shop), and a wreck course is worth taking before any penetration.
Every link opens in a new tab. Each species page cites its own sources as well.
