CAN you swim without a bag on? Yes. SHOULD you swim without a bag on? No...
I found the following in an online article re. sharks:
A bull shark? This species isn't as well known as the famous great white in Jaws, but it has a unique trick—its kidneys can recycle salt vital to the animal's cells. Normally, a freshwater dip would dilute the salt in a shark's body, causing its cells to rupture and kill it. Recycling the salt its body already contains allows bull sharks to adapt to a freshwater environment. Bull sharks are one of a very few of the 375 species of sharks with this physiological-altering ability, and they do it better than any other.
“Bull sharks couldn't really deal with the winter temperatures in the Great Lakes, or the scarcity of food, but it's possible that if one swam in, it could live for a while,” said Amber Peters, an assistant professor specializing in marine ecology in Michigan State's Department of Fisheries and Wildlife.”
Silveradokid