Make sure you fight with clean water which may be either tap water or those popular large jugs of water. And ensure that the water is not hot. Although water of room temperature should be fine, but depending on how hot it is outside pouring ice cold water on the people should not be a problem as long as you are not striking them ice cubes. More so, you can fight with water pistols, water buckets, cups and with garden hoses for that matter. High pressure hoses and guns are, on the other hand are prohibited for safety reasons failing to adhere to which those may be confiscated and you may be charged with a heavy fine for using them