This one swaps the studio floor for the Charles itself: a stand-up paddleboard becomes your mat, launching from Paddle Boston’s Charles River Canoe & Kayak base. Morning sessions catch the river at its calmest, when the water is glassy enough to hold a pose without fighting the chop.

Expect the fundamentals of a flow class made honest by a floating platform — every lunge and balance posture recruits your core in ways dry land never does. It suits confident swimmers who like their workouts with a view; plan on getting at least a little wet.