A room without any noticeable things, which is converted into a room with a pool at the push of a button. Another push of a button causes the pool to disappear again without a trace.

In the picture you can see the redstone circuit and the strip of water, in real use you would sink the whole construction into the ground and build the outer wall of the room above the strip of water.
In deactivated mode, the floor of the room is a smooth, empty surface (made of wood in the picture). On the wall there is a button with which the floor is lowered with pistons at the places where the pool should be later, as well as the strip of wood that separates the later pool from the permanent water strip at the edge. The water from the water strip now flows to the pool area. Since the water strip goes around the corner, the "fountain effect" is created, whereby water source blocks are created at all points in the pool.
If you press the button again to make the fountain disappear, the blocks in the middle are simply lifted again and the water disappears.

Instructions:

The Redstone circuit is too complicated as I would like to write it here in text form completely. So I will only mention the basic elements of the Redstone circuit. The button is connected to a toggle circuit (with a blocked redstone amplifier). This toggle circuit controls the pistons under the pool. In order for the pistons to all receive a redstone signal, there must be an area of redstone flares below them. The redstone flares at the edge can now be controlled with amplifiers. To get to the flares in the middle, another floor must be built down, again creating an area (but smaller this time) of flares. These can again be controlled with amplifiers.

The partition between the pool and the water strip is also connected to pistons. These are not connected to the toggle circuit, but directly to the switch. However, a storage cell with timing must be built in between, otherwise the pistons will not stay retracted long enough for the water to spread.