A tide change has hit the Whidbey Adventure Swim this year, its fourth as an open-water race.

Run by the South Whidbey Island Masters, a group of swimmers dedicated to the open water, the race is based at Seawall Park in Langley.

The course, completed in either four laps or eight —- 1.2 miles or 2.4 miles —- is new this year. It was designed to stay closer to shallow water, allowing racers to see the seafloor as they splash past one another in a triangle.

“If somebody needs to stop, they can stand,” said race director Emily Weinheimer, adding that at its most shallow, the water will be about 4 feet deep.

Only 50 people can sign up for the swim, which has had the course record set each year. The participant limitation was necessary to keep the open water just that —- open. Unless a swimmer signs a waiver saying he or she is capable of taking on the frigid water of Saratoga Passage with just skin, they are required to wear a wetsuit.

In addition to needing a U.S. Masters Swimming membership or one-day permit, racers will need a steely calm as tranquil as the water’s surface.

Racing in open water is different from pool races in one major way. There are no lanes, so everybody thrashes and kicks and reaches into the same area at the start. It’s also different because without lanes, the swimmers’ trajectory is far from an efficient straight line. They bow out and in from buoy to buoy.

“It takes some getting used to,” said Jeff Jacobsen, a member of South Whidbey Island Masters, who will race in the Adventure Swim.

Each year, a course record has been set. The current best times are 27:58 and 29:50 for the men’s and women’s 1.2-mile course and 51:40 and 1:00:12 in the 2.4-mile race.

A high tide of about 8.2 feet is predicted by 10 a.m. the day of the race.