If this is not possible your horse should be watered, before feeding, at least 6 times a day in the summer, 3 times a day in the winter.
Standing water becomes unpalatable so change water twice a day.
Make sure troughs / water buckets / automatic waterers are kept clean, do not allow green algae to build up.
A hot, tired horse or one who has been deprived of water should not be allowed to gorge themselves, frequently offer small amounts until thirst is quenched.
There is no need to deprive a horse of water before work.
Horses will sometimes become dehydrated due to heavy sweating, body salts should be replaced by adding electrolytes to water or food.
If possible offer slightly warm water in winter and to horses who are hot and tired.