A diving trip to the Coral Sea - what could be more thrilling? But there was just one problem: first you had to travel 150 miles out into the open ocean to get there. Sue Ditchfield recalls the worst night of her life

'So, what do you do for a living?" asked the cheery voice behind me. I turned to answer, muttered a few incomprehensible words and was immediately sick again over the side of the boat.

Five hours into a 15-hour boat journey from Cairns to the Coral Sea, at the start of a diving trip, I had already lost count of the times I had been sick. I stared stoically into the darkness in front of me, grasping the side of the rolling vessel as it headed straight into the waves. It may only have been a 20-knot wind and a "moderate swell", but to me it felt like an Atlantic storm in full force.

Most of my fellow passengers had long since retired to their bunks, and were presumably sleeping - though how I'll never know. Two minutes in my bunk had convinced me that I could never sleep there. It wasn't so much the fear of falling out as the sickening feeling of weightlessness every time the bows dipped. It was, as another passenger pointed out the next morning, akin to being put on the worst fairground ride imaginable - and being forced to stay there for 15 hours.

In the cockpit there was at least fresh air. There was also the comforting presence of the boat's captain, Gavin, who attempted to engage me in conversation, fetched me water every time I was sick, a pillow to rest my head on and even gave me his seat behind the wheel. There I spent most of the night, slumped, head on pillow, feeling worse than I ever remembered feeling before.

"A trip for those who think they've dived the best, who want to be spoilt forever..." the brochure had promised. I had envisaged drifting through clear blue waters, in a kaleidoscope of corals and fish. Or relaxing on deck, wine-glass in hand. What I had never considered was the reality of heading 150 miles out into the open ocean.

The hours passed in a nightmare-like trance. I was vaguely aware of other people appearing, exchanging a few words with Gavin and disappearing again. I tried to lift my head to talk, but the result was another quick dash to the side of the boat. I felt incapacitated, like being blind-drunk without any of the preceding enjoyment. Physical action was impossible, but my mind was racing. "Get me off this boat," it yelled. "Get me back on dry land. Now. Before I die."

I thought of the pioneers of past ages who had endured endless journeys across unknown oceans; I thought of the impossibility of being rescued if you fell overboard at night; in my unbalanced and melodramatic state of mind I even thought: "Better jump and end it all now."

At 2am the cook, Sam, came to relieve Gavin. She found me a bench to sleep on and I snatched a few uneasy hours sleep. By 9am we had entered the reef, and the sea had calmed considerably. I forced down some breakfast and by 10.30am was somehow ready for the first dive of the day.

Things got better. I succeeded in doing eight out of the planned nine dives. And it may have been worth the suffering to experience the spectacular shark-feeding dive. But there was still the return journey...