Crazy! I was drownproofed as an infant, and was a water baby my whole life. I joined the swim team in highschool and university. That swim test was stupidly easy if you knew how to backstroke. Just hold the weapon above the water in both hands, and kick. Your head will dip below the water, but will come right back out, so breathe then and exhale while your head is under the water.
They made me do it side stroke as well. That was much harder, but I could have kept going for at least 200m (down and back 4 times.)
I had no clue that us competitive swimmers have that much more endurance in the water than the average swimmer.