Well, the protecting part kind of negates most of the other requirements. An actual personal protection dog is a LOT of dog. Even a bite sport dog is more dog than most people want or need.
On the other hand, it sounds like what you really want is a medium to large-ish dog that looks intimidating enough to deter the casual scary person (a serious scary person would probably just shoot the dog and be done with it). In which case, I'd suggest that you look for an adult black Labrador Retriever. They are big enough to be intimidating, research shows that black dogs "look" scarier than other colored dogs of the same breed/type, they have a big bark, Labs will (typically) eat any and everything, usually love swimming, and have dense, double coats.
On the other hand, it sounds like what you really want is a medium to large-ish dog that looks intimidating enough to deter the casual scary person (a serious scary person would probably just shoot the dog and be done with it). In which case, I'd suggest that you look for an adult black Labrador Retriever. They are big enough to be intimidating, research shows that black dogs "look" scarier than other colored dogs of the same breed/type, they have a big bark, Labs will (typically) eat any and everything, usually love swimming, and have dense, double coats.