A swimmer in Sydney Harbour passes over a shark. Credit:Sydney Institute of Marine Science He's seen all the harbour's splendours, and has glimpsed its dangers: the three-metre bronze whaler that ripped a client's kingfish out of the net as he bent over the side of the boat to bring it on board; two large tiger sharks (one longer, he claims, than his six-metre boat) lounging beneath his craft one day as he fished off Balmoral beach; the mako that took his line and led him a chase across to Manly before it leapt from the water and broke free. The channel takes a gentle turn and we're heading towards Sugarloaf Bay, a pretty name, forever tainted by tragedy. The bush-lined cove must have seemed an idyllic setting for 32-year-old actress, Marcia Hathaway, and her journalist fiance, Fred Knight, as they anchored here for a lunchtime boating party with friends on January 28, 1963. Wading in 75 centimetres of -water, they would have had no hint of danger. Yet soon after they entered the water, a large shark attacked, leaving Hathaway's right leg almost severed. Within 20 minutes her massive wounds had proved fatal. Newspapers of the day reported the incident in graphic detail: Knight's heroic struggle to wrench her from the jaws of the creature; his swim to a nearby house to call an ambulance; the mighty effort the party made to push the vehicle up the slope when it broke down on the steep bush track. Her dying words and the shockingly candid photos of her, open-mouthed, being stretchered through the bush - these words and images burnt themselves so deep into my brain as a young child living in Sydney that I had nightmares for months afterwards, and the seeds of a life-long phobia were sown.

A great white shark near Pulbah Island in Lake Macquarie. Credit:Rod Collins Strangely, the episode helped me become a champion swimmer in primary school. Early each weekday morning I would arrive at the harbourside Redleaf pool ( now re-named the Murray Rose pool) in Double Bay, and swim at top speed between its two pontoons, imagining what could be pushing through a hole in the shark-proof fence. These days, I'm still a pretty good swimmer, but despite the teasing of friends, I can rarely be coaxed much past neck-depth at the beach. Every year, I vow to join one of those ocean or round-the -harbour swims, and fail to find the nerve. Berg refers to such events as "smorgasbords" - for the sharks, that is. The day grows overcast as Berg motors towards the mangroves at the far end of Sugarloaf bay. It is difficult working out where the attack might have taken place, though sand is just visible underwater next to the bank in some places. Berg's depth sounder tells us the drop-off is steep: 11 metres very close to shore. Middle Harbour is a drowned river valley with some of the deepest holes in the harbour - up to 45 metres – providing an environment ideally suited to large fish and their predators. The day after Hathaway died, a hunting party hooked a large bronze whaler very close to where she was attacked. Berg believes it was the same shark, and that he may even have seen the culprit the night before the attack.

You couldn't give me a briefcase of money big enough to swim to the other side. Myer Berg He and his father were fishing from a sea wall near the Spit Bridge when a woman next to them, using a hand-line and dangling her lower legs in the channel, felt a disturbance below the surface. She shone a torch in the water to get the shock of her life. "Right under her feet was this massive shark," Berg says, insisting he remembers the encounter "clear as a bell" despite being so young. "It was heading up this way and Hathaway got bitten the next morning; I think the chances are pretty strong it was the same one." There have been at least 16 fatal attacks in Sydney Harbour since the late 1880s, all in the summer months of December, January, and February. Hathaway was the last to die. But Paul de Gelder came very close in February 2009. The navy diver was on a dawn assignment at Garden Island, helping to test sonar gear and swimming along the surface on his back when he felt "an almighty whack on the leg". He turned to see what he described in his book, No Time for Fear, as "the huge grey head of a bull shark ... [ with] the upper row of its teeth across my leg". Twice it pulled him under, shaking him like a rag doll as he tried to punch free. De Gelder lost a hand and a leg and only immediate access to first-class medical treatment saved his life.

Berg is taking us around Middle Head now and over to Watsons Bay, where we rendezvous with a boat run by Berg's fellow guide and great mate, Craig McGill. McGill says neither he nor Berg were surprised when the attack on de Gelder happened. The harbour was "full of sharks" that year because of the huge quantities of kingfish in its waters. Indeed the pair had been laying bets on where - not when - an attack would occur. A few days after de Gelder was hit, Balmain resident Louise Keats reported seeing a large shark leap from the water at Mort Bay, near her home. It's a place where I've often seen children jumping from ferry wharves in the hottest days of summer. I'm horrified to find in the Fairfax archives two newspaper reports, one from 1895 and one from 1929, detailing the grisly deaths of two young teen boys in Balmain and Rozelle, both taken by sharks, one after he leapt from a jetty just as local kids do today. Berg says he often sees children swing into the water from a Tarzan rope near Beauty Point in Middle Harbour, near a deep channel where he catches large fish. The day after our harbour tour, he texts me to ask whether I've seen that day's news: a 2.5 metre Great White caught in nets off Bondi beach. Outside the heads, admittedly, but still, a little too close for comfort. I ask Professor Emma Johnston, head of the Sydney Institute of Marine Science, what she feels about the risk of harbour swimming. She says bull sharks in particular are a presence in the harbour in the summer months, travelling large distances - one tagged specimen lingered in the harbour for 14 days, swimming more than 230 kilometres at night and 65 kilometres during the day. The tagging program run by the NSW Department of Fisheries shows they can sometimes be in close proximity to swimmers, divers, kayakers and boaties.

"We know they are there,"Johnston says. "We know they are moving in and out and not eating people who swim. But I think that points to a couple of things. There are ways of minimising interactions with sharks which don't remove all risk, but do reduce it. Not swimming at dawn and dusk, and not in highly turbid water. Bull sharks are tying to target fish. But if it's murky and there's thrashing about and they are undecided, and it's feeding time, then mistakes" - as she delicately puts it - "can happen". She says she is far more worried about gastro bugs in the harbour after heavy rain than sharks, and swims regularly in open harbour waters around Chowder Bay . After all, there are probably tens of thousands of Sydney-siders paddle-boarding, dive-bombing, swimming, kayaking and climbing out of boats into the harbour every year. But the fishos aren't persuaded, though they do agree with her on the riskiest times to be in open water: at dawn and dusk in the warmer months, and when bait fish are abundant. "Sharks are not actively feeding during the day and I think that's why we get away with so much swimming in the harbour," McGill says. "If there were that many people in the water at dusk, dawn or at night, I reckon it would be carnage." So if it's a bright sunny day, the water is clear, no one is chucking fish guts off the pier and kids are doing a few water-bombs into the sparkling depths, I'll stifle my misgivings. But you'll only catch me in the harbour on the landward side of a shark-proof fence.

Department of Fisheries - Tips to reduce risk of shark attack: - Don't swim too far from shore. - Swim in groups. - Avoid being in the water when it's dark or during twilight. - Avoid murky water, waters with known effluents or sewage.

- Avoid areas used by recreational or commercial fishers. - Avoid areas where seabirds or diving or there are other signs of baitfish or fish-feeding activity; Sharks may be present between sandbars or near steep drop-offs. - Avoid swimming in canals, and river/harbour mouths. - Avoid swimming with pets.