All depends on how you hydrate before the activity and how long you will be continually active in the water. Therefore if you properly hydrate before and you don't exceed 60 minutes of activity, you probably don't need to drink more than 3oz of fluids every 15-30min during practice/meets. All told, other than the fact that swimmers wouldn't have the same sweat losses as other athletes training it hot weather, they should basically follow the same athletic hydration guidelines shown below:
General Fluid Requirements
• Fluids should be cold, palatable, selected based on the type and duration of the activity.
• Sports drinks should contain 4-8% carbohydrate. Drinks greater than 10% carbohydrate may slow stomach emptying, cause abdominal cramping and impair performance.
• Drinks with a combination of glucose, glucose polymers and fructose may enhance water absorption.
• Solutions containing primarily fructose can cause an upset stomach and should be avoided. Be sure to check the food label for ingredients.
• Drink approximately 16 to 24 ounces of fluid two hours before activity.
• On warm or humid days, drink an additional 8-16 oz 30 to 60 minutes before activity.
• Water is adequate for activities less than an hour as long as meals are consumed regularly.
• For endurance events, training sessions longer than 60 minutes, or multiple practices a day, choose a sport drink containing four to eight percent carbohydrate (e.g. Gatorade).
• Depending on your sport, consume 3-6oz of water or sports drink every 15 minutes. This equates to approximately 32 ounces per hour.
• For prolonged exercise greater than 60 minutes, choose a sports drink with small amounts of electrolytes.
• Immediately following activity, drink at least 16-20oz of fluid for every pound of weight lost to ensure proper rehydration.
• A liquid shake with high carbohydrate content, minimal protein and fat can refuel energy stores and maximize recovery after demanding training bouts. Consume this as soon as possible after workouts or events:
• Drink an additional 16 ounces with your post workout meal. This meal should be consumed within two hours after activity.
• Weigh yourself each morning. A fairly stable weight generally indicates proper fluid balance.