I bought SPF 30 and slathered my kids with it when we lived in Virginia and that was fine. I bought the same strength when we moved to Puerto Rico. There was a swimming pool about a block down the street from where we lived and we went across the base so that they could take lessons in a large pool. Toliver was three at the time and fearless. He would jump in the pool in water over his head even though he could not swim, so he needed lessons. Callista, who is three years older, swam well but I knew she could swim even better if she took lessons as well. We never went to the pool during the hottest part of the day and we were fine.
The base also had a lot of neat little beaches and our favorite was Crab Beach, even though Toliver said it as "Cwab Beach" so that is what we called it. We took a picnic and went swimming there one day. The next day, I saw that the kids had little blisters on their shoulders. Yikes! They got sunburned becausetheir skin was just dark enough that I couldn't tell it was burning.
After that, I always made sure that the kids wore white tee shirts when we visited the beach and I beefed up our sunscreen to SPF 50.
I did not realize that my husband could tan. When we lived in San Diego, CA and went to the beach, he never wore sunscreen. He got a tan! Granted, it took a while. My kids were very dark when we lived in Puerto Rico, even though I went through gallons of sun screen. I guess the sun is just so strong near the equator. They do even though we are living in Maine again and the sun is a lot weaker. You can't be too careful with skin cancer.