Well, if you have a car, you could easily take a break, go through a drive thru and come back.That way you wouldn't need to change to eat.
I'm pretty sure you can also bring in coolers as long as you have no glass containers. You could pack a lunch that way.
You could also just do what I usually do- eat just a snack there and then schedule an early dinner. Pizza or a fruit cup or even doughnuts or a frozen hot chocolate at Blizzard Beach, or a grilled cheese pretzel or a deli sandwich or wrap at Typhoon Lagoon. A nicer restaurant near the park entrance would be a great idea for one of both of those places- one with an outdoor area for wet swimmers and an entrance outside the turnstiles so people could eat there even without a ticket. However, it is extremely rare for water parks to provide anything more than simple snack-lunch type fare.
Enjoy!