You totally missed my point. I'm not against the idea of making a hole and filling it with water, but I can't agree with the way you are trying to sell it: "a pool that is meant for swimming that increases happiness", because that idea seems useless to me.
Now, making ponds as in digging some land and filling it with water sounds like an interesting idea, but there are some problems with this: if the pond or "pool" is made of shallow water tiles it means you can pick water from it, so you could make a pond next to your house and in turn make wells completely useless. So, this pond would either need finite water (as in, it can dry up) or a new kind of water tile which is meant for swimming but can't be used to fill buckets.