Yup, like all the PPs mentioned each of the parks and DTD has a splash/wet area.
MK- Casey's Splash Area
Tiki's/Totem's in Adventureland (they more-so squirt you)
Epcot- On the bridge between WS and Futureland
AK-Near Kali Rapids
DHS- Inside Honey I Shrunk the Kids
DTD- Next to Earl of Sandwich and between Christmas Store and Design a tee. (these may not be both on at the same time depending on time of the year and if they have the dance party next to EOS or on the stage!)
We always make sure we have a swim diaper and change of clothes for our DS. A towel would definitely help dry off and warm up (water is sometimes FREEZING), but we don't always bring one.