THE SIGN READ "CURIOS AND BRIC-A-BRAC". I was clearly the first customer for months. A brass bell clanked mournfully on its rusty spring. The shop and its contents lay under a fine shroud of dust. Dim fingers of light perforated the gloom through windows laced with grime and cobwebs.
     I knocked over a heap of Victorian photographs. Grave faces stared up at me. I felt as if I were trespassing in a stranger's tomb.
     A dull glint caught my eye. In a corner, wedged between an umbrella-stand and a mounted gazelle's head, was a hardhat - a cumbersome globe with a brass-barred window of greenish glass.
     "Just the £10, sir," spoke a reedy voice. A wizened creature shuffled into the light and peered at me through gold-rimmed pince-nez.
     "Done!" I replied, surprised by my own impulsiveness. And thus, God help me, did the Haunted Helmet come into my possession.
     In all likelihood, it would have remained in my living room and never again seen service under water had I not broken the strap of my mask as I packed for a diving trip.
     Hastily fashioning a neck seal from gaffer tape, lard and clingfilm, I bundled the helmet into the boot of my car and set out for Wales.
     I was at 20m off Skomer Island when I first became aware of its extraordinary powers. A pair of wrasse sidled by, and one said to the other in a rich, Welsh brogue: "Diu, diu, diu! An' what do you call that?"
     "I call that a prat in a hat, Barry," replied the other.
     "I dunno. I mean, is it really worth it, dressing up like a complete pillock, just to get insulted by the likes of you an' me?"
     "They're oblivious, innit? Watch this!" The fish turned to me and shouted: "Oi! You big nancy! Why are you wearin' a goldfish bowl on your 'ead?"
     "Mind your own bloody business," I retorted. Seldom have I seen a wrasse so stunned, except when they dynamited the reef at Hurghada. They stared at me in blank amazement, then vanished into the murk.
     That was just the beginning. I saw a choir of mackerel performing Verdi's Requiem and a hermit crab complaining to a parasitic estate agent that he'd been gazumped over a whelk-shell in St David's Bay.
     I overheard a sea cucumber suggest a symbiotic relationship with a voluptuous pink anemone (she slapped him with a tompot blenny).
     "Listen, matey," hissed a dogfish to a polymog, "don't you rattle your sodding spines at me! I'm a shark, y'know!"
     "Technically, perhaps," replied the imperturbable polymog, "but you are to a great white as a pygmy shrew is to a rhinoceros. So you can bugger right off."
     "Haddock is the new cod," opined a haughty representative of the former species.
     The codfish laughed. "That'll be the day! The cod has always enjoyed a special place in the hearts of men."
     "So did the dodo," replied the haddock. "You're fished out - official. Nobody gives a stuff about quotas, so you'll be extinct in five years."
     "So will humans if they have to survive on a diet of haddock. They'll die of boredom."
     "Boredom?" exploded the haddock. "You can call a haddock lots of things, but boring isn't one of them!"
     "Hah!" spat the codfish. "You could bore an arsehole in a wooden horse!" At this point, I decided to terminate my dive, and ascended disconsolately to the surface. After all, if I'd wanted bigotry, aggression and sexual intrigue, I could've saved the petrol money and gone down to the pool on club night.