People used to go swimming in the nude. When is this sefer from? maybe it’s from that tim frame and therefore it owuld be assur. But a bathing suit is not assur.
If I’m reading MR correctly, the sefer doesn’t talk about swimming but seeing one’s wife. The swimming is extrapolated from that (since people generally don’t wear tznius bathing suits).
But then again, I could be wrong.
In any event, I disagree with him in his entirety on the subject.
The Wolf