Do rats live in my pond Or is there something that looks like a rat swimming underwater?

It's unlikely that rats live in your pond, as they typically prefer terrestrial habitats. However, what you may be seeing swimming underwater could be a muskrat or a similar rodent, which is often found in aquatic environments. Muskrats are larger than rats and are adapted for swimming, with a streamlined body and webbed feet. If you're unsure, observing the animal's size and behavior can help determine its identity.