Different situation but the same principle applies here as does to the "blade of grass" scenario.
The question is whether this OB shot is played from spot A or spot B. The correct answer is of course, A. The reason it is A is because where it struck the tree before falling into the OB water was over the OB area and thus OB. The whole tree itself isn't in bounds because it is rooted in bounds.
Same thing applies to a blade of grass. Just because it is rooted in bounds does not mean the whole thing is in bounds. The portion of the blade hanging over an OB area is considered OB.
This seems a little weird. I understand fully but it technically would have to mean that the tree top is considered OB. By real land definitions the tops of the trees which extend over dividing lines that cloud up division should be trimmed or removed and considered a part if the ground. If a disc goes through the tree but kicked back out and fell into the water how do you know for sure it didn't touch in bounds? The tree IS in bounds....?
To me the tree technically needs to be coming out of the pond as that is the dividing line on OB and where the tree would need to be rooted. The branches are not hanging freely.
We "judge" OB planes high up in the air all the time -- where did it cross the plane of the fence, road, or pond; did that thumber make the mando, etc. Your card can make the call as to whether it penetrated far enough to be over in-bounds before it kicked out. But that doesn't make hitting the tree anywhere safe -- not by any stretch