MOWSA currently offers several solo, sanctioned swims that challenge marathon swimmers over long distances in cold water.

Boston Light Triple (BLSx3) – 24 miles (38.6 km)
M Street Beach, South Boston to Little BREWStER Island, Back to M Street Beach, and Back to Little Brewster Island

The Boston Light Swim and the Boston Light Double have a ton of history. The BLSx3 is the new kid on the block, first completed in 2019 as a proof of concept swim; we weren’t sure we could time the tides right or have enough gas left in the tank to battle the flood tide on the third leg, but with strong swimmers and good conditions, it’s absolutely possible.

The BLSX3 is a tide-assisted, cold water swim. It starts from M Street Beach in South Boston, winds though the Boston Harbor Islands (which is a national park) out to Boston Light on Little Brewster Island. The Double is completed when the swimmer returns on a flood tide along the same course, back to the M Street Beach. The Triple adds one more 8-mile return to Boston Light on Little Brewster Island so the swimmer can land in the shadow of America’s first lighthouse.

What to expect

Water Temperature: 58 to 70 degrees F depending on date

Tides: This swim has to be tailored to the swimmer's pace. You start at the M Street Beach in South Boston with the ebb tide and need to reach Boston Light on Little Brewster Island within 30 minutes of slack and swim back to South Boston on the flood tide. On the third leg, swimmers typically have about about an hour or two of flood tide they must swim into before the tide turns and provides assistance to the finish.

Weather: Anticipate ocean swells, wind, waves, and overcast skies. You may get glassy water or four-foot swells. This is New England; expect it all.

Swimmer notes: The swim starts from a sandy beach. Little Brewster has a pebble/rock beach. The finish is on Little Brewster island and because of the nature of the landing there with rocks and with the water potentially being higher than when landing a double, swimmers are permitted to stand in knee deep water for an official finish.

The water temperature is coldest at Little Brewster Island. At each turn, the swimmer is allowed time to reapply sunscreen and anti-chafing products before starting the next leg, but you should not linger too long--officially you have just 10 minutes to get underway or the swim is terminated. Plus, standing around wet after exiting cold water can quickly lead to hypothermia for most people. 

You'll be in the open ocean for the duration of the swim, so expect to encounter some marine life. Recreational and commercial traffic is also present, and your pilot will do their best to communicate with other mariners and navigate potential hazards, but be aware you may be asked to wait to permit a ferry to cross; commercial traffic has right of way on this course.

Dates with favorable tides in 2026: TBD

MOWSA runs solo swims only on weekdays to reduce the impact of recreational traffic. The BLSx3 starts in the early morning at a specific time in the tide cycle to capitalize on that assistance, which means there are limited opportunities available each season.

Solo Records

Banner photo by Elaine K. Howley