Every transport aircraft I've had experience with have all had spring loaded fuselage drains located at several locations along the belly. These drains are spring loaded open when the plane is not pressurized to allow any fluid (water, fuel, hyd fluid, lav juice etc..) to drain. They automaticly close when the aircraft is pressurized.
Now, assuming the aircraft remins intact during a ditching, the second cabin pressure is equalized, the drains will open and allow water in. Yes, the plane may bob around for a few minutes, but I guarantee it'll sink straight to the bottom soon thereafter.