Workplace (Health, Safety and Welfare) Regulations 1992 say that an adequate supply of wholesome drinking water should be provided for everyone in the workplace, regardless of whether the weather is hot or not. It also says the employer has to provide cups, unless it is a drinking jet. In the case of non-disposable cups, a facility for washing them should be provided nearby.
Even if you are working outdoors you have a right to drinking water. Water should only be provided in refillable containers where it cannot be obtained directly from a mains supply. Such containers should be suitably enclosed to prevent contamination, and should be refilled at least daily.
If you have a drinks dispenser make sure that it is regularly cleaned and tested for contamination, as they can easily become breeding grounds for bacteria. If your water cooler is supplied through renewable plastic bottles, be sure that whoever replaces them has had manual handling training, as the bottles can weigh up to 20kg.