Presidential decree 1984-320 carved 1,383 hectares of Banderas Bay seafloor into federal protection, creating Mexico's smallest national marine park around five granite islands that breach the surface eleven kilometres south of Puerto Vallarta's hotel zone. The formations — El Pulpito, La Corbeteña, Los Arcos proper, and two unnamed spires — are erosional remnants of the Sierra Madre del Sur range, their sea-level bases riddled with tunnels and arches that funnel tidal surge and concentrate pelagic nutrients. Upwelling currents from the bay's 1,800-metre trench deliver cold water rich in plankton, sustaining reef systems at latitudes where coral typically struggles. The result is a compressed vertical ecosystem: brain coral and sea fans below twelve metres, sergeant majors and parrotfish in the shallows, brown pelicans and magnificent frigatebirds nesting on the guano-stained summits. Los Arcos Puerto Vallarta snorkeling tours operate year-round, though visibility peaks between November and May when seasonal thermoclines stabilise and sediment settles. The park's western margin drops sharply into open ocean, creating microclimates where manta rays and occasional dolphins pass through temperate corridors. Tour operators launch from Mismaloya, a former fishing village five kilometres north, where the 1964 filming of "The Night of the Iguana" introduced international attention to the coast. The shortest crossing — twenty minutes by outboard panga — reaches the archipelago's eastern flank, where wave action has carved the signature twin arches that lend the park its name. Most snorkeling routes circle the northern islands, avoiding the exposed southern cliffs where swells exceed two metres even in calm weather. Conservation enforcement tightened in 2018 when CONANP audits revealed anchor damage to 14% of surveyed coral heads. Mooring buoys now restrict boat anchorage to sand channels, and the three-dollar per-person conservation fee — collected at embarkation — funds quarterly reef assessments by Universidad de Guadalajara marine biologists. Spearfishing, shellfish collection, and reef walking carry federal penalties under Article 420 of the Código Penal. The park's compact size concentrates visitor pressure: on peak winter mornings, fifteen to twenty tour boats may operate simultaneously within the protected zone, making early departure windows the practical choice for encountering the site before midday crowds and chop arrive. Snorkeling tours from Puerto Vallarta typically bundle the marine park with nearby Mismaloya Beach or seasonal add-ons like bioluminescent plankton viewing, though the core attraction remains the same — cold upwelling water, granite tunnels, and the improbable density of life they sustain.