I've thought about the same thing. Our town water (water from the tap which is provided by the local government) is hard-water as it's sourced from an large inland river system so the calcium content is higher. Even so there's way too little calcium in the water to even be a problem for chickens or be enough for layers so it should be safe for your chickens.
Your best bet is to provide them oyster shells as a supplement if you have a mixed flock. Egg shells can also be used but as long as you're consuming heaps of eggs, there just isn't enough to go around. For instance, we consume about 2-4 eggs per day and we have 4 layers and I struggled to provide them enough shells. Oyster shells have a greater percentage of absorbable amount of calcium and so your layers will consume very little so your shells will last a lot longer.