"Make some noise for the crew," bellows an excitable guide. "No noise, no lunch!"
Sixty people in bathing suits start clapping, cheering and whooping. Oh no. This is what I was afraid of.
I'm in Cancun, Mexico's Vegas by the sea, and have signed up for this "Paradise Island" boat tour for what I'd hoped would be a respite from the town's all-you-can-drink happy hours and pole-dancing waitresses.
But this doesn't bode well. An armada of minivans has delivered us to a dock north of Cancun where we've been divided into groups and allocated boats. I'm in the English/French group and our guide is an enthusiastic Mexican called David.
"Is anyone celebrating anything?" he cries, once we're all aboard. "No? OK, let's celebrate being alive!"
I let my head slump onto my life jacket. It's going to be a long day.
Our first activity is snorkelling over a section of the Mesoamerican Barrier Reef, the world's second largest coral reef. With Bob Marley's Could You Be Loved blaring from the stereo, we skim up the coast, past palatial waterfront homes and a marina brimming with super yachts.
Once we reach the reef, we're kitted out with the requisite gear and given an unexpectedly thorough safety briefing. David is particularly adamant that we stay on the surface. "No diving," he says. "And no standing up. We must not damage the coral."
Another surprise is that we're joined by an inspector from the marine park who swims with us to ensure we follow the rules.
It's an overcast day and the visibility isn't perfect but we still see an enormous variety of marine life, including brightly coloured parrot fish, huge groupers and a nurse shark lurking in the shadows. Almost as impressive is the variety of coral, from lace-like fan coral that wafts lazily in the current to spherical one-metre-high brain coral that is hundreds of years old.
Back onboard, we speed towards Contoy Island, one of Mexico's most important marine bird refuges. The narrow eight-kilometre-long national park is home to more than 150 species of birds and is visited by more than 10,000 every spring. Only licensed tour operators are permitted to land and numbers are capped at 200 people per day.
"Welcome to paradise," says David, as we pull up to a wooden jetty in a sheltered cove. And, to be fair, it is idyllic. A curve of pure white sand fronting a shallow, bath-warm lagoon fringed by coconut trees.
Lunch is served in an open-air dining area and clearly we made sufficient noise earlier because it's a feast. While we tuck into barbecued chicken and grilled fish with guacamole and salad, a manta ray glides through the turquoise shallows in front of us.
Most people look as if they'd be happy to spend the afternoon lazing on the beach but David encourages us to come on a tour, which turns out to be the highlight of the trip. To my surprise, beneath the amiable, fun-loving exterior lies a knowledgeable and passionate environmentalist.
As we stroll along a boardwalk into the island's mangrove-choked interior, tiny hermit crabs scuttle into the bushes while bright green lizards eye us quizzically from the undergrowth. David points out a boa constrictor curled around a tree branch and we watch dozens of frigate birds soar effortlessly over a crocodile-infested lake.
We finish the tour in a small museum that contains models of some of the turtles, sharks and rays that can be found in the surrounding 5126-hectare marine park.
Before we return to the boat, David gathers us round. "My friends," he says, earnestly. "Today I would like you to be super heroes. There are two things you can do to help save the planet."
He then launches into an impassioned plea for us all to stop using plastic bags, which can kill turtles and many other types of sea life, and to not use sunscreen while swimming in the ocean. He tells a heartbreaking story of a tourist who left a sunscreen-covered handprint on a centuries-old brain coral. Two months later it was dead.
It's an unexpectedly heartfelt speech and it's clear that he genuinely cares. "I want to create some conscience," he explains, "so please spread this message around."
Back on the boat, it's business as usual as we speed towards our second stop, Mujeres Island. The music is cranked up and the crew oversees a drinking game with tequila (which, admittedly, is a lot of fun).
Mujeres comes as a rude awakening after Contoy. The beaches are busy, the streets are lined with bars and souvenir shops and tourists whizz around the island on golf carts.
We have an hour to explore, which is just enough time for a stroll and some shopping. Before we get off David makes one last request: "My friends, please don't buy any shells or starfish. They belong in the ocean, not in our homes."
Latin America specialist Chimu Adventures can create a tailor-made Mexico itinerary including flights, accommodation, transfers and tours. Phone 1300 773 231; see www.chimuadventures.com
Rob McFarland was a guest of Air New Zealand and Chimu Adventures.