I see the problem with draining being that to visualize the trip it would help to think of twenty swimming pools in a row each connected at a different height with a tube you can swim or crawl thru depending on the water level. Some of the pools are full, some of the tubes are full. Some you crawl thru, some you swim thru. Just because you drain some of the pools closer to the pumps doesn't mean that you can also drain the other pools. Some of the tubes you have to push your scuba tank ahead of you to squeeze thru.
Oh, and fill the whole thing with chocolate milk instead of water.
