It was like being in that film, Deep Blue Sea, the one where Samuel L Jackson gets bitten in half whenever I walk into the room. You sit there, dangling your simple bait... and something pulls a little, and again, and harder, and more... and then you pull on the rod and the meat's gone. You are, in truth, mainly feeding piranha rather than fishing for them.
And then comes the moment. The bite, the pull, the excitement from out boatman - and up it comes, my piranha. Ten inches long and teeth like razors. I don't know why I thought they were small. A vicarious and atavistic pride fills me and I want to eat it, vegetarianism notwithstanding, but they put it back for someone else to catch later, an act of mercy which rather pleases me until that afternoon when I find myself unaccountably swimming in the same lagoon.
It was Nelson, our native guide, who jumped in first, but it seemed churlish to leave him to be eaten alone and anyway it looked so cool. The piranha, he insists, stick to their own end - apart from the little tiny ones. They gather around swimmers who become even slightly still and nip.
Not enough to make you bleed and bring the big fat angry ravenous one who hates you because you stuck a fish hook in him dashing over, fortunately. Anyway, it was a lovely swim, and Nelson said it was safe because the cayman alligators only come out at night. It was only when we got out, tired but happy, that a cloud passed over the sun and a 16-foot cayman alligator swam the lagoon from one side to the other, right under our noses.
So what would you all have said if I'd been eaten alive? But he told me it was safe, and he was doing it. Where have I heard that before?
Dr Selby is a GP in Suffolk.
You can write to her at GPcolumnists@haymarket.com.