Although boats are permitted on St. Mary Lake [1], there’s no place to rent them. Windsurfers sometimes take advantage of the ever-present breeze here, generally wearing wet suits because the lake never seems to lose its chilly edge.
Anglers generally prefer boat fishing to bank fishing on St. Mary Lake, which is not really known for good fishing but does have some whitefish, rainbow, and brook trout. Hikers are rewarded by better fishing at Red Eagle Lake, south of the park entrance, or at Gunsight Lake.
Links:
[1] http://www.moon.com/destinations/montana/glacier-national-park/logan-pass-st-mary/sights/st-mary-lake