Out here in An Tir, 'Heavy' water bottles are not permitted to be used on lights. You can, of course, approach within (I think it's something like 20 feet?) and yell "Water!" at the light. The light must then acknowledge the drink, and retreat from the field to the rehydration line before returning to combat.

I've heard in some kingdoms that 'Light' water bearers are allowed to offer water to heavy fighters, but only after their initial supply of light water is exhausted. Is this the case where you are?