as a former scout leader, In my group all participants (over 400 now) had to have either their bronze level certificate, equivalent of it, or pass a swim test that demonstrated equal skills. All trips were also announced several months in advance to allow time to acquire those skills. Over the years I had several kids that either had poor or no swimming ability, my leadership team worked with each to MAKE SURE those youth met the standard.
In 20 years, I've only ever excluded 1 youth who refused to even try (We even taught a few that were hydrophobic). Proper planning and enough time to acquire those skills are what's needed to ensure a safe trip. Those kids also learned navigation, rescue (self and others), emergency 1st aid, wilderness skills, etc.
In the bush, with a 6 to 1 ratio, and all the activity; in 1 minute, you can have tents going up, fire lit, food prep, gear sorting, and myriad other things happening, all requiring adult supervision, and kids being kids, one can easily slip away and fall in without notice. To add non-swimmers to the mix is courting disaster!