This hat, purchased at Costco, can be used for water play. It has a neck flap and the inside is mesh, so I think it won't be hot to wear. The size is indicated as 3 to 8 years old. There is a string on the outside to adjust the size. Measured without tightening, the head circumference was about 52 cm. Since this is a rough measurement, there may be some error. I don't remember wearing it since I bought it, so it's almost like new and unused.