Yes, the supply of extra water is necessary in warm / summary weather.

• supply lukewarm water, 1.5 - 3 litres in the milk period, depending on the calf's age
• pay attention to where the calves are accommodated. Consider shadow areas, isolation of the roof (cooler shed), air movement (cooling).

It is possible to state: water should be supplied at all times. If calves can drink water sooner / earlier, they will also start to eat roughage and concentrates sooner / earlier. 
This will stimulate the rumen at an earlier stage,which will have a positive effect on the calf's health.

Good luck with creating sustainable dairy cows!

Also read the article "Water supply of vital importance".