Some of our kids have trouble drinking water. No one knows why, although, it has been suggested that maybe it is a sensory issue. Here on some thoughts on ways to get around the problem.
Add Water to Food
I dilute everything Erin drinks and some of her food, too, like soup and chili. She gets enough water through these liquids and foods. She is now drinking 8-10 ounces of water before every meal. She says she can't stand the taste of water after she has eaten food, so she drinks water before. That's good for appetite anyway. I think we're doing pretty good water-wise, but it's because we have agreed upon a plan, not that she drinks water in response to being hot when she's playing outside or whatever.
Something that has never stopped working for us is getting a few ounces of water in with yogurt....if yogurt is already on the snack menu like it often is for us, we just turned it into a drink by spooning it into a cup and adding a couple ounces of water or by blending it up with crushed ice in the blender.
We use rice, soy, and almond milk, all diluted 50-75%. If I dilute it a little less than usual, Erin will say, "Mom, This milk is a little sweet. Are you paying attention to what you're doing?" There are some brands that have fortified or extra calcium options. We try to get those to add more nutrition along with the fluids.
Adding flavor, texture and changing the temp of the water all help to directly address the sensory component. In our case the issue very much exists (when Nick was a toddler he could drink juice no problem but would open his mouth before water was swallowed because he couldn't taste or feel it was in there and it would run out of his mouth). Reving up the water with these different tricks helps circumvent that sensory problem.
One thing that I do do occasionally, however, is cut up a cucumber and put that it in a pitcher of water in the fridge. It looks so pretty and special and my kids like it. It isn't sweet, but it is a refreshing and distinctive taste. Lemon and lime slices or rosemary alone or with the cucumber are 2 more of our favorites. It's fun for the kids to freeze mint leaves or edible flowers in ice cubes and then they try to drink the water all down before the ice cube melts into the glass...little tricks can be a big help!