The first item I always need in my beach bag is an oversized towel that I can lay out on or dry off with.
Another thing I can't leave at home is a spray sunscreen. I really like the spray because if you do go to the beach, sand doesn't stick to you as much because it is fast drying. I also choose one that is 30 SPF or higher!
If I'm by the water, I want my belongings to be protected, so I always bring a watertight box just in case.
For after the beach or boat, I like to bring a travel sized shampoo and conditioner in case I am going somewhere afterwards.