= I normally do my swimming in a pool. In the sense that when you swim, you do it in a pool and not in the ocean, for instance. If this were purely a statement about the fact that you swim regularly, we would normally just say “I’m a swimmer”, and WHERE you swim would be secondary information. However, if that issue were pressed, you would answer with “I swim in a pool.”

= I’m swimming at this moment. THE pool if it’s yours or a pool known to you and the listener, e.g. the pool at your gym, the local public pool, etc.; A pool in any random pool, e.g. a new friend’s pool, who the listener doesn’t know yet, a pool in a hotel while on a business trip, etc.