Hi Heidi,
Welcome to the Disney Parks Moms Panel and thank you for asking us a question! May is one of my absolute favourite times of year to visit Walt Disney World Resort, so I think you're in for a real treat.
While water fountains aren't marked on the Theme Parks Guide Maps, you can definitely find some throughout the Theme Parks and Water Parks! I've usually seen them around the
Restrooms, and those are definitely marked on the Guide Maps.
One minor caveat... While I'm definitely not a huge consumer of bottled water when I'm at home (largely for environmental reasons), I've often find that the water in Walt Disney World Resort isn't that great tasting. I've found that it has a bit of a sulfuric smell to it, especially in the hot summer months (May should be better). Because of that, I do tend to consume a fair bit of bottled water while visiting the Theme Parks. You can buy it on-site but, if you're looking to save a bit of money, every Walt Disney World Resort Hotel room includes a mini fridge. I often buy a case of water (either online at a retailer like Garden Grocer or, if I rent a car, at a conventional local grocery store) and bring a few bottles with me for my family for our day of fun!
Please don't hesitate to come back and visit us if you have any other questions about your upcoming visit to Walt Disney World Resort!
Have a Magical Day,
Lynne