Carrots and apples are about 90% water. For comparison the primary feeds that are fed to horses, including baled hays, feed mixes and hay pellets, contain approximately 10% water. You did not indicate how many of each are fed but if you are like most horse owners the amounts fed vary from a couple to several.
Bottom line: no worries! I view carrot and apples as treats and an opportunity to bond with your horse. I hear the stories about too much carotene, too much sugar, and horses becoming to aggressive and nipping. The water content of the carrot and apples and the amounts fed make these treats a non-factor relative to nutrition. When I was a kid I would have a peppermint in a work coat with an oversize side pocket. Jody-Grey was the gelding I rode (big ol’ quarter horse) and he would go into that pocket to recover the peppermint. He was conditioned to only look once and only when I turned to him with the pocket side containing the peppermint. I did not always have a sweet for him and if I did not turn for him to look he knew the pocket was empty. He never nipped because of the conditioning he received..