Technically speaking, a One Day One Park ticket does not entitle the use of our transportation system (with the exception of TTC <-> MK). In reality, we tend to look the other way, so to speak.
*HOWEVER*... there is no way to go from the MK to a water park directly. If you have a car, my advice would be to use it. If you are being brought in by an offsite shuttle company, that complicates things.
If the drop you off at Epcot, Studios, or Animal Kingdom, there will be buses that can take you to Blizzard Beach only. Typhoon Lagoon requires going to a resort, and then catching the DTD bus from there. (Old Key West is usually the best transfer point for that area)
Word of advice... if you'll be traveling during peak summer times, beware. Water parks have been going Yellow or Red Flag often times by 10:30. This means the park is full to capacity, and NO ONE will be let in. (If Yellow, only Disney Resort guests may enter)