Pumping water from closed reservoirs.

Sometimes you need to excavate an area that is flooded. Currently, the only way to drain it is to fill it completely with dirt. Why not add the ability to pump water out from bodies of water that are not connected to the global ocean? You could build a dam around the area and pump the water out. This would also be useful for dealing with flooding in mining pits.

That would require water simulations, there are many suggestions for this already.

I don't think it'd be too taxing to detect when a body of water is enclosed and convert it to a lake of sorts. Then using seawater pumps to empty it. It's probably take awhile, but still simpler than filling it with rock and subsequently removing it.