Try not to get pool water or seawater on the extensions. I know this is near impossible but follow these tips to ensure you have no problems.
Always wear a hat when sunbathing. Tie your hair back and bring your ponytail around to your front so that you are not lying on it. If you can, apply a sun-factor to your hair. When swimming keep it tied back. After swimming untie and spritz with a leave-in conditioner and gently run through with fingers or use a wide toothed comb. You can make your own spritzer with a spray bottle of water with some of the recommended conditioner poured into it.