My DnD group recently had a scene where we were escaping a tunnel system under a lake. My character used a bag of holding to slow drain the lake until we were at the bottom near the entrance of a tunnel. Picture a sink with the drain at the bottom. The tunnel system basically snakes down and up like an exagerrated W, with a pocket of air at the top and us starting on the right top side.
My idea was that I should be able to elevator us down and then up each bend by swallowing / releasing water from the bag. The DM figured that the water on the other side of the bend would stay up and collapse onto us as we reached the bottom of the first bend. How would the water behave in this scenario? Does it depend on whether the space between bends is air-tight or not?