Ideally, you should leave 1-2 hours after eating before you get in the pool – but for early morning swimmers, this can be easier said than done! When you’re already getting up at the crack of dawn to swim before work, losing an extra hour in bed to have a hearty breakfast may not be achievable (or desirable!).
If eating a proper breakfast isn’t an option before your morning swim, you should try to at least have a small snack. You may not feel like you need it, but it’s especially important to refuel after your body has been fasting through the night.
Regardless of what time you choose to swim, the key is opting for easy to digest carbs that will release energy throughout the course of your swim and ward off any sudden sugar crashes.
Source: https://www.swimfreak.co.uk/blog/diet/what-to-eat-before-and-after-swimming/