That depends on who you believe and what you are trying to achieve with your food intake. The word diet, in this case, means "what you eat"; everyone is always on a diet. Many people go on special diets from time to time to lose weight or body fat, gain weight or lean (muscle) weight, or maintain weight (among other reasons).
What is the best diet for you - how much fat, protein, and carbohydrate - and how to decide what will work for you? The answer is up to you, your physician, and your personal needs.
This article will touch on some of the current ideas in nutrition, and some of the ways those apply to swimmers. You should consult your physician before beginning any type of specific diet to avoid or reduce the chance of medical complications.