As a baby for outdoor swimming, we always put sunscreen on , there are so many baby/children ones to choose from. She would also wear a one piece UV swim suit with long arms and down to the knees also a hat with peak and neck cover. Now she is nearly two she has a two piece UV suit and will wear the same sort of hat.
To be honest I thought I would dress her in the cute little swimming costumes but the sun is so strong we save those for indoor swimming.