Swimming

Sweating and dehydration is not something swimmers generally consider, after all, you're surrounded by and immersed in water, so how much can you sweat? The question can be partially answered by looking at the number of times swimmers get cramp during a training session or a race.

Whilst cramp sometimes occurs due to improper warm up, generally they are a direct result of sodium depletion. In a swimming pool you won't notice sweating as much as you would in the gym, simply becuase you're already surrounded by water, and any salts that are sweated out are immediately washed off the body due to the flow of water around you.
A 2% loss in body weight due to dehydration can reduce performance by more than 10%, whilst losing more than that can result in loss of consciousness. If this happens in training, not only will your training and recovery be much less enjoyable, but the effectiveness of the workout and potentially the next workout could be reduced.

Racing

After hours of hard training, it's not uncommon to see athletes ruin all the hard work by failing to properly consider their nutritional needs on race day. One common mistake is getting through the heats then relying on catering at the event, where more often than not you're lucky to get a decent burger.

There are three main factors to consider come race day.
1. Energy Provision
All olympic disciplines of swimming are heavily dependant on the carbohydrate energy system. If you are to swim at your best you will need to ensure that your nutrition contains adequate amounts of carbohydrates to fully replace your muscle glycogen stores.
2. Hydration
It is well documented that as little as a 2% loss of body mass due to dehydration will cause performance to fall by 10% or more. Dehydration can be a major factor in swimming, especially at galas, due to the humid nature of the environment. Pool sides are hot and sticky at the best of times, by the time all the competitors and spectators are there the temperatures soar, so sweat rates are likely to be far above normal.
3. Maintaining Familiar Nutrition
It's best to check what food will be available at the race venue and, if you're travelling, accomodation before you leave, rather than find out there is nothing you like when you get there. It's always worth taking some food you know you can eat, and that you know you can compete on. Training on the food you will race on is always recommended.