Combine all of the ingredients in a blender and blend until completely smooth. If your batter seems too thick to pour out of the blender, add more water 1/4 cup at a time-- if your dog loves ice as much as mine does, it doesn't really matter if the flavor gets diluted a bit. (The icier the treat, the longer it lasts!)
Arrange a silicone mold like this one onto a sturdy baking sheet and pour the batter into each crevice, making about a dozen treats. (It's important that the silicone mold is on a sturdy baking sheet BEFORE pouring the batter into the mold, as the silicone is very flexible and could cause the batter to spill when you transfer it to the freezer.) Freeze until firm, about 2 hours, then pop the little treats out of the mold and store them in an airtight container in the freezer for up to 6 months.
These treats can clearly be customized as you see fit, and would also be a great way to sneak any supplements into their diet. (We give our dog a cranberry supplement to help with urinary tract health, so sometimes I'll add that in, too.)