IF YOU can't run, walk. If you can't walk, do the Australian crawl. But whatever you do, don't stop.

Australia's fastest growing sport, ocean swimming is a siren song for athletes whose bodies bear the hallmarks of a lifetime in high-impact sport, giving them a second chance at competitive recreation.

Derek Mortimer, 73, began ocean swimming when his knees gave out after he exerted himself in fun runs well into his 60s.

''For years after I stopped running I used to have these wonderful dreams where I was running, and running very well, better than I do in real life. Now I have them about swimming,'' he said. ''I couldn't run, but I could swim without any pain so I was able to replace one sport with another.''

Between October 2010 and June last year, 94 ocean swimming events were listed in NSW and ACT - up from 17 in 1999, according to the Ocean Swims director Paul Ellercamp. More than 17,000 swimmers competed in NSW alone.

''People who run, cycle, do triathlon, often come over to swimming after the jarring and stress of those sports has worn down their bodies, particularly their joints,'' Mr Ellercamp said.

For Libby Stewart, 66, a semi-retired physiotherapist, ocean swimming is different from swimming laps because the changeable nature of the water means different muscles get a workout every swim.

Her husband, Richard Stewart, is director of the Palm Beach to Whale Beach swim, which has 2000 entrants, most of whom fall into the 40-plus age category.

Mrs Stewart, a self-proclaimed ''mad sportswoman'' who surfed and played tennis, overcame her fear of the deep and pushed out into ocean swimming about 15 years ago.

''The point about swimming is it's the one sport where you've minimised your body weight and gravity so it's excellent as an adjunct to other sports,'' she said. ''If I wasn't swimming I'd be in trouble. I had to retire from physiotherapy because of my rheumatoid arthritis.''