Mice are interesting mammals that have many surprising capabilities when it comes to travel. They can climb walls and easily get upstairs in your home, and they can scamper around so fast that many people barely notice them.
Mice are very adept swimmers that are capable of crossing waterways and treading water for hours or days at a time. Their bodies are light and have relatively thick fur, so they can survive swimming in less-than-ideal conditions.
You can certainly find mice in a large pool. They might climb into the water for a drink or think that they have to cross the water. The downside is that mice often die in pools because they can’t get out of them.