Yes, squirrels can indeed swim.
Squirrels frequently visit open water to drink during hot weather, and, in the course of their travels, will undoubtedly come across water on many occasions. Hapless squirrels also sometimes fall into water and have to swim to safety. It is doubtful that they intentionally go for a swim after all, what would be the reason?
How do squirrels swim?
Squirrels swim with the body submerged and only the head above water. When once more upon land, squirrels don't look like natural water-lovers, more like dejected drowned rats with rather odd tails.