This is the question that we had to answer a week or so ago when the boys went swimming at the kiddie pool at my mom and stepdad's house.
We started out with a regular swim diaper on each boy, but Nick quickly soiled his, and I was surprised to find out I'd grabbed a stack regular pull-ups instead of extra swim diapers. They're the same color, so it wasn't too crazy of a mistake.
Now I see why swim diapers are so necessary. Nick's pull-up just kept getting bigger and bigger. I think by the time we gave up he could barely stand up! It would be an interesting experiment to see how much water those things can hold. The boys both had a great time playing with Grandma and Grandpa, though.
Henry did surprisingly well in the little pool. He kept trying to get in and out, but he did an incredible job of managing that all by himself.
My mom had fun using the squirty balls to get both boys wet. Nick found a squirt gun and had a ball trying to squirt the squirrels and the dog.
A wet time was had by all.