Start when they're little and don't ever stop doing it. Always have a decent backpack, drinks bottle, footwear, socks, hat, sunblock, first aid kit, microfibre towel and technical layers ready to go at a moment's notice. And food. Lots of food.
If you stop doing it, they turn into the child who hyperventilates as soon as there's more trees than concrete. Especially if Daddy's new friend 22 and very easily impressed also tells them they're always dressed like a boy, it's really cruel for Mummy to not let them be a proper little girl and why on earth does Mummy want to do boy things like climbing rocks and riding horses at her age (37) and muscles are just so unfeminine, maybe Mummy wants to be a boy?
Rant over, it was ensuring that they were never uncomfortable, wet & cold for more than a couple of minutes, hungry or stuck doing exactly the same thing all the time - the most important things being the feeding and planning in times for them to chill out at home, play with friends (Saturday afternoons with random equipment worked really well, as the little group turn up with things like cricket sets, supersoakers, Nerfs, pavement chalks, yo-yos, etc, which also saved having to buy absolutely everything) and eat. Did I mention that catering is essential?
Getting cold and wet from swimming was a good thing if it was followed by warm, dry clothes and a portion of chips from the chip shop eaten waiting for the bus and a hot chocolate when they got home. Getting rained on and blustered about whilst out was always acceptable if it meant a doughnut and a hot chocolate in a cafe (or in the back of a car with a dryrobe these days). Parkrun would have been brilliant had it existed at the time, as it would be meeting other kids, dogs and having the hot chocolate/sandwich/ice cream afterwards.
You will need to pay attention to food and snacks. I might have suggested this previously, but in case I didn't make it clear enough, they will need a supply of food and drink that can be handed to them as soon as the first 'I'm hungr-' starts.
The hardest thing is getting inactive people, especially adults who have never done anything like it, to get onboard and keep doing things until they become a habit.
I would strongly suggest, OP, that you take swimming lessons yourself, though. It'll make all of this so much easier for you, rather than making it something entirely dependent upon your DH.