What's actually glowing
Microscopic plankton — dinoflagellates — produce a flash of light when the water around them is disturbed. It is a defence reaction: the flash startles whatever bumped them and draws attention to it.
That's why it responds to you. Kick, and the water around your legs lights up. Trail your hand and it leaves a glowing wake. Pour a handful and it sparks like static.
The moon matters more than the month
People ask which month is best. The more useful question is which night. Bioluminescence is visible much of the year in the bay, but you only perceive it when it's genuinely dark — so a night near the new moon beats a full-moon night in the 'best' month.
Cloud cover helps for the same reason. So does distance from the city: the glow that is invisible off the malecón is obvious once you're out by Los Arcos with the lights of town behind you.
Intensity does vary night to night — it depends on plankton concentration, which nobody controls. Anyone who promises you a guaranteed light show is selling something.
How the night runs
Our trip leaves timed to sunset, so you get the golden hour going down the coast past Mismaloya, a stop at Colomitos while the light fades, and then full darkness at Los Arcos — which is when you get in the water.
Snorkeling in the dark sounds unnerving and isn't. You're in warm, calm, shallow water with a life jacket, the crew in the water with you and the boat two metres away. The disorientation lasts about ten seconds, until the first time you move your arm.
What to bring
A towel, dry clothes to change into, and a sweater or light jacket — the breeze after dark is cooler than you expect coming off a warm day. Leave the camera expectations at home: phones essentially cannot capture this, which is part of why it stays memorable.
