They usually have a handle on top so you can lift them out easily if they fall in. Also if the banks are steep the dog cannot get out, or if it gets caught in a current on in weeds it can get in trouble.
Most dogs are natural swimmers, as are most humans. Only thing is, like humans, many of them dont know it.
But, as Lankeela says, it's not just a matter of swimming. A dog overboard in a steep-sided river or canal has little chance of getting out without having a handle to grab.