firstly, you need to buy lots and lots of vaseline and then cover yourself in it.
Second, put on your goggles and swim cap and jump in and start swimming around the perimeter of the pool.
the action of you swimming around the perimeter will then create a whirlpool and then due to the force and the all-mighty power of the whirly-pool the mattress will magically float out.
You're welcome, chump