A swimmer that is training heavily (more than two hours a day) should eat four to seven meals (3 major + 4 small) a day.
The amount of endurance it takes to be a good swimmer is why a swimmer's nutrition is so crucial to their strength and speed in the water.
It takes a good three hours for your body to actually digest the food you’re eating, particularly all major meals, so if the swimmer has eaten an hour before he/she hits the pool, his body is trying to digest and it can’t deliver the blood flow to the muscles, so he ends up getting a stitch.
Overall calories requirement of swimmers is always more than normal human being and it depends on multiple factors like training hours, height, weight and gender.
If their nutrition needs are not getting full-filled through food and its vital for them to nourish through supplements for growth, health and good condition.
Recommended foods pre/during/post workout
Make sure that you are fully hydrated and you had good carbohydrates in breakfast/lunch.