(blow on the check valve and see which side works?)
But you don't need a check valve...
Check valve is only required if there is pressure on the water/cable, which if you got the cable going in from the top, is not the case.
Shut you air pump off and watch the cable.
You will see it raise to water level en then stop.
Depends, if it has air bubbles in the sponge it can float.
But floating is not what it suppose to do, weight it down and keep it down.