Standing DOES NOT lower the center of gravity of the combined rider/bike system
It raises it. However, what it does is decouple the movement of the body CG and bike CG by allowing them to move somewhat independent of each other. This is a great help on rough terrain. However, on a smooth slick water crossing, sitting vs standing doesn't make much difference. If you watch real close, you'll notice that all the guys that go down are not perfectly straight up and down AND going straight. They are still slightly leaned and trying to complete the corner when they hit the slick stuff. OR, they get on the power while on the slick stuff. Even if the bike is perfectly straight up and down and you are going perfectly straight, grabbing even a little bit of power can make the rear slide right out form under the bike. Even trying to hold a constant throttle is risky because it can make the back end break loose. Trying to change direction AT ALL can make the front tuck or the rear slide out. The trick for this is to be straight up, standing or siting, and coasting across in a straight line. Even then, if there is enough water current, even that can push the wheels out from under you or cause you to need to make a course correction that can put you down. So ideally, you'd want to start on the upstream side of the crossing so that if you do get pushed a little, you'll hopefully be on the far side of the crossing before reaching the down stream side and falling off or trying to correct your direction.