I missed out on that whole 'it's not possible' thing, but I do know there's a mod for it. Actually, the creator Nerdy Doll has already made almost all worlds swimmable (Willow Creek, Brindleton Bay, San Sequoia, Granite Falls etc.).
If i remember correctly they do it similarly to Softerhaze with their lighting mods, by editing the "world data".
If individual creators can overhaul these maps, there’s really no excuse for EA to say it's 'not possible'.
It's really hard to accept such a cheap excuse from EA when independent creators are like already "doing it"... 🙃