I have always allowed my girls to swim (if they want) while nursing pups. Most of my girls love it and it keeps them clean and cooled off. I towel dry them paying attention to the mammary area ( good time to check for cracks or teeth marks which could lead to mastitis). My girls are releuctant to leave the pups for the first week or so....but after that they look forward to a leisurely swim in the afternoon. Never have had a problem.
You're on your own with this as far as I know. I never, ever allow my girls to swim until milk is drying up after the litter is 6 weeks. I know they want to swim but with light chemicals or salt pools or a pond with bacterias, it's a big chance.
Several repro vets have highly recommended against it. I listen and I use common sense along with my club friends.
As someone mentioned the cervix is still open also.
