We have only been waiting in the grey Atlantic swell a few moments when the first flash of metallic blue appears in the water. A blue shark, a few miles from the coast of Penzance in Cornwall, emerges from the depths. It is time to get in the water – but part of my brain rebels.

“It’s not what you think it will be like … not that ingrained fear that everyone has about sharks. But until you get in the water with them, that fear will remain,” the guide says to the group.

Slipping off the boat, covered head to toe in dark wetsuits, we are instructed not to shout if we see one but to raise a hand, wave and point. We wait, peering through the gloom at the mackerel lure below us. But the shark does not return, and we heave ourselves back on board.