There are different sizes...natch. What works on a Rottweiler
wouldn't work on a chihuahua.

My neighbor has two dogs, both of whom love the water. One is a
Rottweiler, an extremely strong swimmer who loves current and will
swim until you drag her out. The other is an English bulldog with
zero buoyancy. Rottie doesn't have a PFD and doesn't need one;
bulldog got one early this summer and has been happy as a clam ever
since. Oddly enough, he really took to rafting, too. The thing
about dogs and boats is it's just like kids and boats: you have to
be prepared for any range of reaction from love at first site to
hate-it-won't-get-in-it. If they hate it, there's really nothing
more to be said: that one's staying on shore, and that's all she
wrote. It's really hard to predict what dogs will take to boating:
I've known strong swimming breeds who hated it and aquatically
challenged ones, like this bulldog, that couldn't get enough of it.
All you can do is try it and see how it goes.

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Mary Malmros
Some days you're the windshield,
Other days you're the bug.