I'd agree a swimming test is as good as anything as a basic fitness test, but consider the two following candidates I've had:
Trainee stuntman with rock climbing, trampolining, horse riding etc as other required disciplines - could do the 400m in 5 mins, but since they were 7 stone wringing wet, could barely put on a tank and getting a person out of the water was 'barely able'.
Tired diver tow - barely scraped the minimum time.
6ft ten man mountain, barely scraped the 400m swim time, yet could lift a diver fully kitted out of the water whilst fully kitted.
Smashed the tired diver times as he had legs like tree trunks.
I know which I'd consider more useful than the swimming test seems to indicate.