There are lots of no no's on swimming diets. For a day I suggest no diary such as cheese, milk, yougurt, things like that! You can eat pasta, that's really good for you! Also drink things that are under 22 grams of sugar or less. Gatorade is good, but propel is the best you can drink. No sweets either, you can cramp up, and loose speed. Make sure you have plenty to drink to keep your deherated. Here is what I eat for my swimming days
Breakfast~ Plain bagel, with NO cream cheese, or butter, and an apple
Lunch~ Pasta with a piece of bread, and turkey, no mayo, and lettuce, and No cheese
Dinner~ Usually we go out after a swim meet whenever you want.
But if your going to like practice, eat a good meal before you go, and not to eat alot, but I hope this helps, and Good luck!