Hi Sabrina,
The dates that you are looking at will be a great time of year weather-wise to visit Walt Disney World. It will be warm, but the heat of summer will not yet have arrived. You are looking at Memorial Day weekend, and this will likely mean somewhat higher crowds, especially over the long weekend; however, it will not be as heavy as some more popular times such as Spring Break or the Christmas holiday. By using
Disney's FastPass+ system to make selections for some attractions with reduced wait times, and by arriving early at the theme parks, I think you will find the crowds quite manageable at this time of year.
As for the Disney water parks, my family and I love to head to one each time we go to Walt Disney World! We love to splash the day away on the attractions and floating in the lazy river. There are a couple of ways that you could purchase these tickets. You could purchase the
Disney Water Park tickets separately and by the day, or you could add the
Water Park Fun and More option to your theme park tickets. Which of these is more cost-effective for you will depend on the number of days that you are planning to visit. By exploring the websites I linked, you can see which option is right for you. While my family and I love the Disney water parks, the Walt Disney World resort hotels also offer wonderful pools. If you are staying at one, relaxing by the pool at your resort is a great option!
I am limited in the amount of space that I have to answer, Sabrina, so I will leave you with one low-cost tip. I like bringing our own drinks and snacks into the theme park with us in our backpacks. This saves us lots of money on snacks inside the theme parks.
Please do let us know if we can help you plan further, Sabrina!