Yes, you must be able to swim 50m un-aided. In our third week of training we spend the day at the Qantas training centre. We had to wear overalls over our bathers and get in the pool with life jackets on. We then had to get ourselves into a T25 life raft and set up the canopy with the lights out and rain (fire sprinklers) on. Then we need to drag a team mate thru the water and swap. Then we need to swim up and back. People have failed because they Havnt been able to complete this part of the training process. But I also know of plenty of people that went and learnt to swim when they discovered they had gotten into GS and passed the day with flying colors
You get out what you put in when it comes to GS
But this particular day is so much fun