Depends on how you are traveling. If you park at the TTC, you will take the ferry or monorail over to MK.....about 15-20 mins. Heading to Epcot? Hop on the Epcot monorail line...takes about 20 mins. DHS?? Bus there, takes about 15 mins, and AK, same thing. As far as the water parks, I 'think' the bus to DHS may get you to TL and the AK bus may get you to BB....but as I've never done it, I can't say for sure.
If you are staying onsite, it is much easier to just take the transportation from your resort to the park you want to be at.