Translation:I swim in the river in the summer.
は marks the topic, not necessarily the grammatical subject. The sentence basically reads 'speaking of summer, X swims in the river' where X is omitted and left implied.
Now you could definitely assume that X is summer, but the much more natural reading (basically how Japanese people would automatically read that sentence) is that the speaker is X, unless there's some other context that makes it clear they're talking about someone or something else. (We don't have that, so Duo expects I or we). If you want to say summer is going swimming, you'd have to go out of your way to make that explicitly clear.
You just need to get used to reading things the Japanese way - if someone said "Summer! Gonna swim!" you'd understand exactly what they meant, right? Same deal