Note that the swimming test is possibly the easiest physical test you will ever do in the Army or anywhere else for that matter.
All you have to do (unless things have changed!) is tread water for a bit and swim about 50 metres - using the technique (or lack of it) of your choice.
Take a couple of lessons first and it'll be a swan.
I suffer from negative buoyancy - or at least I did. There was a picture of me sinking in the Dead Sea whilst trying to do the 'read a book whilst floating' tourist thing. Sadly, my wife stored my photos in a leaking shed and it turned out that the photos couldn't swim either.
My glorious military career was entirely swim test-free, apart from a trying episode in my teens as an RAF cadet where I resolutely doggy-paddled my way to victory, taking a considerable amount of time and subject to the PTI shouting 'Well done Captain Webb' as I finally reached port.