Even for white water kayaking it is NOT essential for someone to be able to swim 50m to do the activity safely.
A number of my friends (and DH) have professional level permits. They all say that it's not an absolute essential skill because you don't need to be able to swim. You need to be able to float and not panic and for the number of experienced members in the group with the right skills in lifesaving to be appropriate for the conditions, the level of experience of the group and the actual number in the group. What they have to be able to argue to their governing body who issues the permit, is that they have allowed for this in their risk assessment.
The issue with booking onto a course or getting paid instructors in, is the ratio of adults (or instructors) to clients is more rigid and fixed (often simply for insurance purposes). They will generally be for eight people because that's how they make money. They couldn't take out eight non-swimmers to one.
But they could take out a smaller group in theory if they risk assessed it properly and their insurance allowed for it.
The biggest issue is having enough people to respond to a situation and not having someone panic. Your general most likely 'worse case' scenario is an 'all in' situation. So you need to be able to manage that and that's what you cater to (and what your insurance will effectively based on). That's obviously easier with swimmers than non-swimmers. If you have a non-swimmer they are much more likely to panic. Saying can you swim 50m is about reducing the risk of panickers to a certain extent.
However experienced instructors will happily, for example, take out their own kids younger than the age limits centres have or below the level of being able to swim 50m.
The point is there is discretion allowed in this and that's different to a hard and fast rule on no swimmers. But centres will almost always lean towards protecting their long term future and ensuring they are well within their insurance requirements (cos that's just sensible).
That's also why Scouts also allow for this discretion. To allow for an appropriate risk assessment and more situation specific scenarios. Precisely so you could be more inclusive and allow entry into the sport. This taking Beavers out with their parents on flat water in a canoe in good warm weather is an option. But you wouldn't take a Beaver white water paddling with Scouting.
DS did first start on mild white water age 6 outside scouting - he had two professional level coaches with him on a warm day when the water was low. It was well within everyone's capability to handle. DS has always been super confident in water even though his swimming ability even now leaves a lot to be desired.
Most instructors I know tended to come from Scouting as their initial entry into kayaking so I do think it's really important to have this flexibility to micro assess the situation. It helps knowing the kids well and knowing their personality (which a paid instructor doesn't have either).
Scouting doesn't tend to get to white white level until you get to Explorer age anyway - and there are very few leaders permitted for it (keeping in mind you need good ratios). It's generally much more entry level. And you also need to have access to the appropriate equipment which is a lot more difficult and limiting (and this is how scouting also covers itself in terms of trips needing to demonstrate they have the right equipment to be covered by Scouting's own insurance / risk assessments). And kayaking/canoeing is limited in terms of where you can actually do it too - you often need permits for the water body if it's inland.
The sea is a little more complex but fine as long as you stick to proper safety and risk assessments.
And tbh every instructor I know is a LOT more concerned about the trend in paddle boarding and the general public going and doing it themselves - often without bouyancy aids. There are far too many teenagers and adults who think they are too cool for a bouyancy aid and don't respect the water. I know of numerous instructors have ended up having to do rescues - at risk to themselves - because their certification basically obliged them to intervene in such scenarios as they have life saving skills. I always worry if we go to the beach with DH for this reason. It's rather less fun being with someone constantly on guard for issues!
In recent years a large percentage of the most tragic incidents in these type of water sports have involved poor risk assessment of the weather and a lack of appropriate safety equipment (buoyancy aids) to the point that paddle boarding has recently been forced to move under the umbrella of regulation of the BCU (British Canoeing) because they have a very good system of regulation, training and safety awareness. It's strict for a reason. But that's about the process of risk assessment itself rather than a more rigid rule on things like ability to swim. Ultimately you have to be able to demonstrate in every situation you are within the limitations of the group as a whole for the settings and conditions on the day.
But not being able to swim itself doesn't stop you being capable of doing these activities at entry level. You just have to adjust accordingly.
I would encourage anyone who is concerned about water activities with scouts to talk to their leaders asking them to run through their considerations for the risks for the group and for non swimmers for this reason.