Hello Michelle and welcome to the Disney Parks Moms Panel!
The weather in Orlando in October is typically BEAUTIFUL- at least in my opinion! The average highs for the first half of October range from 81 to 84 degrees Fahrenheit. For my family, these are ideal swimming temperatures. Additionally, all Walt Disney World Resort pools and water parks are heated, so any dips in temperature in the evening should not have an effect on your swimming comfort. So, with that said, I say go for it!
As for tickets, if you are purchasing the Park Hopper option for your tickets, it is much more economical to add the water park option to your theme park tickets. However, if you are buying non-hopper tickets and you intend to only spend one day at a water park, a single day water park ticket is the way to go. You can look at ticket prices
here and try different combinations to identify the best fit for your family.
If you have more questions, come on back! We love to help.
See ya real soon!
Anne