The Charles separates Boston from its northern neighbor Cambridge, home to the Head of the Charles Regatta (one of the world's largest rowing events), and is a fairly hectic waterway when it comes to boat traffic. Actually, because of the river traffic, you'll have to pass a quick safety test to learn the rules of the waterway before you are allowed to don your lifejacket and play coxswain at Charles River Canoe and Kayak in Boston. The river itself is serene so you'll mostly stay dry, unless the wake of an oncoming speed boat catches you off-guard. Highlghts of the journey include paddling under bridges and seeing the Boston skyline from a new perspective.
It's possible to rent from at least five locations along the river, including Brighton. If you're looking for a really great workout, rent a one-way kayak in Cambridge and drop off at Brighton.