Most mammals with a similar quadrupedal body can swim with a "doggy paddle". Most organs are around the same density as water (and primarily made of water), so just having air-filled lungs and a head/nose that can be lifted up above the rest of the body when it's submerged is all it takes.
In fact, the biggest reason most mammals don't swim often is that they rely upon fur for insulation, rather than fat, and can die of cold, not because they are incapable of swimming.