You also have to be careful since they don’t have a mom to cover them with oil making them water proof. At 13 days old, you are the mom who judges when to end the swimming (ambient temp, weather, water temp, etc.) bring towels so you can warm and dry them off.
Another thing to think about is location. I live in the northern part of Idaho. It’s still cool 40’s at night and 60-70’s day time. That isn’t warm enough even to bring them out side let alone go swimming. Mine are also not going to our pond this year due to Canadian geese who are living there with 6 goslings. I don’t want to expose my goslings to any parasites or avian influenza while they are young with a developing immune system.