DULUTH — Six-foot waves, seasickness and little sleep did nothing to deter six marathon swimmers who on Wednesday morning completed a 48-mile Lake Superior relay from Split Rock Lighthouse to Duluth's Canal Park in less than 24 hours.

"Land feels so good," swimmer Michael Miller said as he set foot on the rocky beach next to the Lakewalk.

The group of cold-water swimmers — who didn't use wetsuits or other assistance — set a distance record for that route on the lake, where the surface temperature hovered around 63 degrees the last 24 hours. The swim was filmed for an upcoming documentary about the effects of climate change on Lake Superior.

Team members each took four one-hour shifts, monitored by crews in kayaks and sail, dive and Zodiac boats. Such a swim in one of the coldest lakes in the world wouldn't have been possible four decades ago, the swimmers said, with lake temperatures in general increasing several degrees over that time.

"Lake Superior is so precious ... but it's changing," said Craig Collins, a 64-year-old Minneapolis resident who finished the last leg of the relay at 7:30 a.m.

Winter ice coverage of the largest, coldest and deepest of the Great Lakes is related to its temperature. And while extremely high ice-cover years like 2014 still occur, Lake Superior is seeing a growing number of below-average years, along with more volatile storms and toxic algal blooms.

On this swim, waves with whitecaps were the biggest challenge, but also "amazing," Plymouth resident Karen Zemlin said.

"You could sometimes catch the top of that five-foot breaker and just feel like a dolphin," she said.

The swimmers are all trained to handle waves that overcome them, she said, managing their breathing to compensate. The turbulence of the swells made some of the swimmers seasick and unable to eat while back on the boat, where they rested when not in the water.

No one slept much on a night when the moon shone brightly.

"It was beautiful," Collins said.

The swimmers kept a rotating schedule, with someone always on deck to dive in. An observer would blow an air horn after each hour passed, signaling that it was time to rotate and drawing cheers. Whoever was awake would help the swimmer who'd just completed their hour with their re-warming process, keeping a close eye to ensure they were OK.

"In a body of water like Lake Superior, you take that seriously," said Zemlin, who has swam the English Channel.

Rewarming involved changing out of a swimsuit, staying out of the wind but in the sun during the day, and drinking something steamy while continuing to move about.

Miller, a Minneapolis resident, called the experience "magical." He said the water had a soft quality to it, different from oceans and other large bodies of water.

Living in mostly controlled environments, our bodies don't get many chances to get out of their comfort zones, he said, "and that's the part of cold-water swimming that gets maybe a little bit addicting because it's very much an endorphin release."

Between them, the six swimmers have 75 years of experience with marathon swims in the Pacific and Atlantic oceans, rivers and large lakes. The group, accompanied by observers, will seek ratification of the swim record from the Marathon Swimmers Federation.

Zemlin said no record was broken, because the federation considers the relay unprecedented. No one has attempted such a swim of that distance and on that route.