The George Bass Surfboat and Ski Marathon runs over six days and 190 kilometres, from Batemans Bay to Eden on the far south coast of NSW.

While there were risks attached to competing in the marathon, it was also both rewarding and fulfilling to finish such a test of endurance successfully, organiser Andrew Edmunds said.

Crews train for months and months in advance, getting out in the six-metre long surfboat two or three times a week in addition to weights and gym sessions, he said.

In the lead up to the marathon's start on Sunday, organisers were keeping a close eye on the weather conditions which were looking "pretty treacherous".

"Day one should be okay but the swell's due to pick up," he said. "Two to three metres for the first day and then it might increase up to five or six metres.

"That's something we'll be keeping a close eye on, it's a bit of a concern."

Organisers keep track of the competitors through GPS trackers fitted to each surfboat and a "flotilla" of support boats also follow down the coast for medical emergencies.

Sharks were another potential risk during the race, being in open water, but Mr Edmunds said competitors and organisers took steps to mitigate the risk, especially during crew changeovers.

In the surfboat marathon, a crew of eight take turns rowing in groups of four and change often.

The nine competitors in surf skis face an even tougher individual trial without teammates to tap.

The marathon is a test of fitness and endurance, Mr Edmunds said.

The race starts at 9am on Sunday. Mr Edmunds said the best place for spectators would be Batemans Bay bridge as the race starts and then from any of the headlands or from a boat on the water.

He hoped for a big crowd to send the competitors off, calling the race, with tongue firmly in cheek, like a "mini Sydney-to-Hobart".