Meltdown happen when you remove the stimulus without warning. Always, Always give a countdown. Either in time or turns.
So reason - i.e. Time to eat now, 10 more seconds before water is turned off. 10..9.8..3.2.1 Turn off water.
The Autism Society do a LIFE changing course for parents on understanding Autism.
Embrace the water games. Make a clear pipe water feature and enjoy to together.
Hope this helps.