The Disney waterparks are cleaner, better maintained and better staffed than any other water park I've ever been to, and definitely more themed than any other water park I've been to. They aren't themed in the same style as MK at all, so you won't likely get characters or that kind of magic, but I definitely feel that I'm in the Disney bubble when visiting Blizzard Beach or Typhoon Lagoon. They have the clean, wholesome, attractive, safe feel of Disney.
Blizzard Beach is the water park with the most and best rides, including a couple of thrill rides such as Summit Plummet. Typhoon Lagoon has a few good rides and the wave pool to end all wave pools (which is actually way too intense for me, even though I've been down Summit Plummet twice- go figure.)
If you decide to visit the water parks, you should set a whole day aside for them. They are real parks- and require a lot of energy to get through, just as MK and Epcot do, if you're actually going on the slides. I don't recommend trying to top off a day at the "dry" parks with a water park visit unless you've been to them before and know exactly what you're doing. And even then, I don't recommend it, as one nice thing about a park with no FP's & no ADR's is that it's a great place to relax, and you'll do that best if you're rested.
If you just want to cool off after a hot day at one of the four main parks, stick to the resort pool. If you are looking for a fun way to spend the day in the water, go to Blizzard Beach or Typhoon Lagoon.