Isla Tortuga and Bioluminescence in One Day, from the Tambor Dock

You do not have to choose. Isla Tortuga by day, the glowing sea by night — one boat, one afternoon on the island, and one long evening that ends with the water lighting up around your hands.

I'm José Cardoza. I live in Santa Teresa de Cóbano, on the Nicoya Peninsula, and I've been guiding both of these trips for years: the island by day, the bioluminescent water after dark. Guests kept asking me the same question — can we do both? Now you can, on the same day, from the same dock.

How the day goes

We meet at the bridge by the Tambor dock and leave at twelve noon. The crossing takes us out across the Gulf of Nicoya to Isla Tortuga — an uninhabited island of white sand, warm water and a reef full of fish. You snorkel, you eat, you swim, you lie on the sand and do nothing at all. Lunch is on us.

Then, as the afternoon cools, we start back. And this is the part that makes the trip what it is: we time the return so that we're still on the water when the light goes.

Somewhere after six, once the sky is properly dark, the sea starts to answer. Every movement of a hand, every ripple off the hull, turns blue-green. We stay out with it for about an hour. Back at the dock around seven.

Why the two halves work together

The island and the glow are opposites, and that is exactly the point. The afternoon is bright, warm and busy — snorkels, fish, salt, food, other people laughing somewhere down the beach. The night is dark and very quiet. Nobody talks much once it starts. People just put a hand in the water and watch.

Doing them on separate days is fine, and plenty of guests do. But there is something about going straight from one to the other, in the same boat, with the same salt still on your skin, that neither one gives you alone.

What bioluminescence actually is

The light comes from dinoflagellates — microscopic plankton that live in these waters and flash when the water around them moves. Millions of them at once, reacting to your hand, the paddle, a fish darting away under the boat. It is not a trick of the moonlight and it is not reflected light. The organisms make it themselves.

It happens all year in the Gulf of Nicoya. What changes is how easily you see it — and that depends almost entirely on how dark the sky is.

The honest part: how much glow you get

About an hour, and only after six, when it's properly dark. That is less time in the glow than on my standalone night tour, and I'd rather tell you now than have you find out on the boat.

That's the trade this trip makes: you give up some of the glow and you get a whole afternoon on the island for it.

The moon matters more than anything else. On a night near the new moon the water is spectacular. Close to a full moon the sky is bright enough to wash a lot of it out — still beautiful, but gentler.

So, plainly: if bioluminescence is the one thing you came to Costa Rica for, take the dedicated night tour instead. It leaves from Paquera, spends longer on the water, and we can pick a night with no moon. But if you want a full day out on the water and the glow as the closing act, this is the trip.

Who it's for

• Travellers with one free day — if your schedule only has room for one outing, this is the one that gives you the most of the Gulf.
• Families — the island half is easy and shallow, and children tend to lose their minds, in the best way, when the water starts glowing.
• Couples — I have stopped counting the number of people who go quiet on the way back.
• Photographers — one warning: phone cameras cannot capture bioluminescence. Not yours, not mine. This one you have to be there for.

A word from a local guide

I have crossed this gulf more times than I can count, in daylight and in the dark, and the second half of this trip still gets me. You spend the afternoon looking at the island. Then the sun goes, the engine drops to nothing, and everyone leans over the side at the same time.

Bring a jacket. It gets cool on the water at night, and nobody ever believes me until we're out there.

¡Pura vida, mae! See you at the dock.

— José