The Boston Triathlon turns South Boston’s waterfront into a race course each summer: a sunrise swim off DCR Carson Beach in Dorchester Bay, a bike leg on fully closed roads including Day Boulevard, and a run through Moakley Park with harbor and skyline views, finishing back near the beach.

Racers can choose sprint or Olympic distances, go solo, or split the legs as a relay, and many fundraise for Boston Medical Center along the way — the event’s community arm also funds free swim lessons for Boston kids year-round. The vibe is more neighborhood block party than elite invitational: Southie comes out to cheer, and the flat, protected course suits first-time triathletes as well as seasoned age-groupers. Spectating is easy, too — post up along Day Boulevard or on the sand and watch the whole thing unfold.