We used Boots Once this holiday. We used the children's factor 30 (or 40, I cant remember) which said it lasted for up to 6 hours with 3 hours worth of swimming time during that period. Perfect. Then we ran out of the chidlren's one and so just used the regular factor 40. We re-applied it after about 4-5 hours but the children's skin was reddish that night. When I read the back of the regular 40 it said that it gave up to 6 hours of protection (same as the children's version) but only about 45 mins of swimming time!!! Thankfully we weren't out in the heat of the day so no harm was done but I'd like to warn others in case they get caught out like we did. The children's was the same price as the other type.