I believe a modder (Arnie) did something that allowed people to swim in Magnolia Promenade in one of his mods, so allowing people to swim in Brindleton Bay and Windenburg should be theoretically possible. I'm honestly not sure how the maps work, aside from having a clickable area that causes you to load a neighborhood.
As for your existing saves, hmm, I think--I'm not a modder--that they load a template for neighborhood areas and save your changes on top, so I'm honestly not sure how changing an existing neighborhood in the template would affect your existing saves. I mean, the little fake houses in that neighborhood aren't really playable space, anyway. I've played a lot in the little starter there, and now have someone living in Venessa Jeong's old house across the street, and... yeah.
Also, speaking of, Venessa Jeong's lot is A LOT bigger than it looks, there's a little fake street running across it, which is interesting! I have some actors living there and built a little outbuilding for their butler LOL.(They don't have Pinnacles money, but can afford the butler.)