Expect calm paddling on the Charles River with skyline views or more open-water feels in the Harbor, depending on where you launch. It's a relaxed, self-paced activity that usually lasts 1-2 hours. You'll get a quick safety briefing, a basic kayak or SUP, and a life jacket. Water can be chilly even in summer, and currents or wind pick up quickly, so it's not completely effortless. Boston's traffic and bridges mean you're rarely alone out there, which feels safe but less wilderness-like.
The best time is late spring through early fall, especially June to September when water temps are tolerable and days are long. Weekday mornings beat weekends to avoid crowds. Expect to pay around $25-45 per hour for a single kayak or $40-70 for a tandem; most places add a small deposit or require a waiver. Full-day rentals run $60-100 if you want more time.
Pick a stable recreational kayak over a tippy touring one unless you're experienced. Skip trying to paddle all the way to the Harbor Islands on a rental – the distance and tides make it unrealistic without a guided trip. Bring a waterproof bag for your phone and a change of clothes in case you get wet.