Deep chest, narrow body, single coat, lack of body fat.
[no buoyancy]
Any dog “built like a hatchet” is going to have problems swimming.
My daughter in law’s family lives on a lake, they have a boxer that will swim for hours, but that is not the norm. I have a Malinois, she will jump off the boat into 50 feet of water like it’s nothing. My old dobe wasn’t much for getting in water either.