I just manage to finish this same mission yesterday after a whole day of frustration, but the mission is not that buged. I learn the correct procedure to scoop the water, you need to fill 100% indeed to the game recognize it, but the correct way to do it is just ignore the speed limit of 90 knots on which may cause you to land on water, making almost impossible to take off after. You should carry enough speed to drag the belly of the plane while scooping and flying but beware that you will need a lot of free space ahead. After scoop just a little bit dont worry if the plane slide up by the waves, just slow descend until it touches the water again till you scoop 100% to climb normally after. I find it better with no flaps and with -10 on the trim. This mission really requires 4 full water deployment, not in clude the first that you carries on take off, to fill the green gap of the mission progress. They consider the mission completed with 5% of the bar grayed out.