Your question made me curious, so I went looking. This is from Yahoo, so take with a grain of salt, but it makes sense so here it is:
In water, flat-faced (brachycephalic) dogs have a greater probability of getting water up their short little noses and sputtering, choking, having trouble breathing, and inhaling water. Many of them have difficulty breathing under normal (non-swimming) situations -- that's why they snort so much, and snore like chain saws (not to mention passing gas). Putting them into water is not a good idea without very close supervision -- and, of course, assuming they really want to be in the water. I would never force one to go into a deep stream or pool, and be careful when bathing them.
Because they are very sensitive to heat, many of the flat faces will enjoy a wading pool that they can walk into, with the water level no more than a couple of inches. That way, if they want to, they can sit or lay down in it with little danger of having it go up their noses. Another good thing to do in warm weather is to wipe the dog's body down with a damp cloth, or lightly mist with a water spray bottle.
Be sure to never leave your flat-faced dog in the car in warm weather -- it doesn't take much to have them go into distress or even die.
Flat-faced dogs are adorable, but we need to take extra care of them to keep them healthy and safe. But they are so worth it!
I also found comments indicating that the barrel chests of many of these breeds make it hard for them to keep their heads above water.
Thanks for the question! I learned something new!
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