If you want to have a fun day without worrying too much, I recommend going for something more rustic and also cheaper. Water on the rides isn't the only worry; there's people spilling drinks, kids running around with ice cream cones, bag hitting things, the chance of rain etc. My perfect bag for such a day would be something with a water proof zipper and chunky leather that doesn't mind the odd stain or rain. Those are the things I'd take into consideration - in the end, everyone's free to make their own mind! Here's to hoping you have a wonderful trip! 
