Getting close to gentle sea giants has had tighter restrictions imposed by reputable eco tour companies in recent years, but that hasn’t made this bucket list experience lose any of its magic.

“You can’t join in if you’re ill.” Warns Juan.

“If you have any perfume or deodorant on your skin – wash it off now.” he continues.

“Leave your jewellery here.” He points to a row of lockers.

“Put your sun lotion on now.”  He barks.

My group of ten fish enthusiasts are with our guide Juan, in the cosy office on main street La Paz. He smiles while he piles on the rules. There are many.

On the boat, anticipation mounts as we speed through white-tipped metre-high waves under a cumulous-clouded sky, warm wind in my hair. A pod of eight friendly smooth grey pacific dolphins greet us with loud clicks as they race along beside us, a giant manta ray gracefully gliding by as part of the welcome committee. The boat slows to a halt, and Juan whispers, “We’re here”.

Peering overboard I see nothing but dark water at first, until a huge grey-with-white-spots head breaks the surface right by the vessel, making everyone gasp. This first glimpse of the reclusive creature makes my heart flutter. Juan edges the boat back to just over the assigned 10ft. The group coos at the size of it. “Oh he’s just a teenager. His name is Flavio.” Juan informs us. This ‘teen’ is 35ft long, weighs five tons (which will double by adulthood) – and I am about to get in the water with him.

“Who wants to go first?” My eyes light up and I am soon descending into the brine.

The water is murky, tepid and brimming with krill and plankton; the reason the sharks are here. I slow my breathing and get used to the gloom, noting the colossal figure of Flavio has turned from his meal to gaze right at me. My heart stops for a second as he surveys me, seemingly deciding what to do. His verdict – keep on eating. Like most whales, baleen-feeding whale sharks sieve plankton through giant keratin fronds. I swim around Flavio to let him dine undisturbed being careful not to touch him – the cardinal rule. As I move, he revolves with me, keeping his dinner plate sized eye on me. It feels like we have a moment. Well actually, about an hour, between scrambling in and out of the water taking turns with the other participants, as only five are permitted at a time. It appears he is almost as fascinated by the odd looking creature in front of him as I am.

After more than one call from Juan, I grudgingly reboard the boat. Zipping back to the real world, I speak to no one, staring back at the site until it vanishes from view.

Did you know?

• Whale shark tourism is very popular and currently worth £77m annually.
• Some countries and tour companies are currently sparking controversial news stories for being too lax with rules on getting close to the fish.
• Eco tours, available in several countries, usually give part of their tour fees to directly fund research into identifying individual animals, studying migration, breeding and feeding patterns, and educating humans on protecting them.