Keys lost in a water evacuation pipe
In desperation, I would likely use a shop vacuum, a wet-dry vacuum that can tolerate pulling out water without damaging the vacuum. It should also handle the small rocks.

Be sure to remove the paper filter if the instructions for the vacuum so state and to empty the bin prior to this operation.

Even if the keys will not pass into the vacuum hose, it may block the hose enough to use as a lifting device to retrieve them.