All dogs can swim, but some breeds do not have the muscling or the conformation to be good swimmers. Yes, sweets can have harmful effects on dogs teeth like they can ours. Sugar and carbohydrates serve as food for harmful bacteria. They produce plaque which can harden to form tartar. Over time, this will continue to develop and can cause damage to the gum tissue, leading to infections.
Dogs have more special cells in their nose and nasal cavity that are sensitve to different odors.
The cells developed over time because ancient dogs where wild and had to hunt their food. They needed this improved ability to smell to track game. There are several reasons your dog may be eating the cat poop. He could have a vitamin or mineral deficiency - dog will often eat poop or dirt if this is the case. And there are others.