In principle this would be possible, but the "gewöhnlich" or "normally" implies that this situation arises often or regularly. So maybe a more probable situation could be when you are travelling somewhere and the only way or usual ("gewöhnlich") way of crossing a river would be to swim.
Do you mean why "man" comes after the verb, even though it is the subject? If so, in German you can exchange the subject with some other parts of the sentence, as long as the (finite) verb stays in the second position. In this case, the sentence emphasises the "gewöhnlich" by putting it in the first place.