Event organisers Eddie Spearing and Jamie Norman said about 325 people would be competing this time, up from 306 last year.
Tomorrow’s forecast is for temperatures up to 33degC, and bright sunshine.
"I reckon we’re going to have the best weather in the country. It looks like there’s going to be low wind and the direction of the wind is favourable," Mr Spearing said.
Six swims are on offer, all starting at Waterfall Creek and heading to or around Ruby Island, which sits a few hundred metres off shore in Lake Wanaka.
This year’s edition will feature the Ruby10, a 10km marathon swim, the only one in the South Island, where competitors will swim four 2.5km laps of the island.
Mr Spearing said while it was not the feature swim of the event, it was a distance that had grown in popularity and he hoped to make it an annual fixture on the national open water swimming calendar.
"It’s an Olympic discipline now so people are doing it more. If it [Ruby10] can become a fixture, there’ll be Auckland, Taupo, [Rotoma] and Wanaka."
The event itself continues to grow year after year, although Mr Spearing and Mr Norman plan to cap numbers in the future.
"We’ve got our own limit of 500 swimmers. There are other races around the country where there are around 850-1000 people. We think it would detract from the race [to have that many people]," Mr Spearing said.
"We’ve actually had requests from people that go to the other big swim events to keep ours boutique and manageable," Mr Norman said.
But both men were stoked the event had grown to the size it has since they came up with the idea over a beer a few years ago.
It is not the only concept they have thought of that has come to fruition.
Tomorrow will also mark the day they launch a crowdfunding campaign for their new Ruby Fresh wetsuit.
Mr Spearing, a design engineer by trade, said the suit was designed specifically for swimming in freshwater, by changing the buoyancy areas, making them warmer, and providing colourful arms that make swimmers "flash" when in the water, therefore alerting boaties to their presence.
Two competitors in The Ruby will receive a free wetsuit by luck of the draw at the event afterparty.
