Years ago, Captain Matthew Webb was the first person to successfully swim across the English Channel. During his swim, he ate raw meat and drank beef tea, brandy, and cod liver oil (YUK). Then later, Gertrude Ederle, the first woman to swim across the Channel, ate chicken, chocolate, sugar cubes and drank beef broth. Today most marathon swimmmers "drink" their carbs and protein. I tested several brands during my training. I finally found the brand that works well for me. I use the products by "Hammer Nutrition"---- Sustained Energy (powder) and 100% whey protein. I mix these powders with water prior to my swim. I tested these products several times during my ocean training. Trying something new on the day of a marathon is a major mistake. I would also recommend having canned peaches (a favorite among marathon swimmers) to help soothe your mouth and throat from the bitter taste of the English Channel.
If you have any questions about sports nutrition, I would highly recommend contacting "Peak Performance MultiSports, Marginal Way, Portland, Maine. They are experts in their field. They were a tremendous resource for me when I started my marathon swim training.