Keen is not the same as able
Some Boston Terriers like water — they will splash in a paddling pool, bite at a hose stream and wade happily on a beach. Whether a Boston likes it, though, is not the question that keeps him safe, because physically this breed is badly designed for swimming. Treating every Boston as a poor swimmer, whatever his enthusiasm, is the correct starting point.
Two features decide it. The Boston's short, flat muzzle means he must hold his head high and work hard simply to breathe while paddling, and those wide-open nostrils sit close to the waterline. His body adds the second problem: a compact, muscular, front-heavy build with most of the weight in the chest and head, so in deep water a Boston tends to tip nose-down and sink at the front rather than floating level like a spaniel or retriever.
What safe water fun looks like for a Boston
A shallow paddling pool in the garden, deep enough to stand and splash in, is ideal Boston water: cooling, playful and self-limiting, because he can simply step out. On a beach or lake shore, keep him in water that reaches his belly at most, on a lead or long line, so a sudden drop-off or a chasing wave cannot put him out of his depth. For boats, docks or any place where falling into deep water is possible, a life vest is not optional equipment for this breed — it is the thing that keeps his head up and gives you a handle to lift him straight out. Fit it snugly and let him wear it on land first so it does not startle him.
Sessions stay short. A Boston who is panting hard, snorting more than usual or whose paddling is getting splashy and vertical is a dog who needs to come out now, not after one more minute. Brachycephalic dogs also overheat quickly, and a hot day plus swimming effort is a risky combination; swim in the cool of the day and rest in shade between splashes.
Pools, and the fall-in risk
Garden pools deserve a specific warning for Bostons. A curious Boston who slips into a pool cannot tread water for long and often cannot climb out at a sheer edge, so an unsupervised minute is enough for a tragedy. Fence the pool, cover it, or keep the dog away from that part of the garden entirely — choose one and apply it without exceptions, including when guests are over and doors stand open. The same applies to ponds with steep sides on walks.
Letting him enjoy what he can do
None of this means a water-free life. Most Bostons adore the shallow version: a pool of their own, a sprinkler to stalk, wet sand and rock pools to investigate on a lead, and a towel game afterwards. What they should never be asked for is a swim-out retrieve, a jump off a dock, or an off-lead run beside deep water. Putting that limit in writing for the people around him matters, because a well-meaning friend may not know the breed's risk, and KinsLe Family Sharing keeps your Boston's shallow-water rule on his profile where family and walkers will see it before they head to the lake.