Fireballs disappear in Water and appear when you delete the waterspot (you can see smoke and waterdrip particles in the picture)
No - they dont disapear - they only hide. if u punch them they explode.

How to test in your own World:
First summon a Fireball:
/summon Fireball ~ ~ ~

Set Water above the Fireball
Punch the water
oh - and sorry for the creepy English, iam German

Hope this wasnt the 1000 duplicate,
froggy