In our backpack we pack a zippered plastic bag (the ones comforters come in) and pull it out and put my purse, valuables and backpack in it on water rides. It has a handle so DH just straps it to his legs. One year we went with brother-in-law and his family, and they were all teasing me about my "backpack essentials" calling me Mary Poppins....that all ended day 2 when my sister-in-laws purse got soaked and ruined their camera

The rest of the trip they even helped carrying the backpack, and said they would never doubt me again!