From the queen of gently-used baby stuff herself, Angie of Freepeats:
Here's a hack for ya'… I was at the beach this weekend with my mom. I was about to tell her how much I liked her beach bag when I realized it was my old tote-style diaper bag. The bottle-sized pockets are perfect for continuous-spray sunblock bottles, the waterproof compartment is great for wet suits, etc., and the myriad of little pockets keep watches, phones, wallets, etc. from getting sandy and/or lost.
Related: Lots of good beach travel tips at Parent Hacks.