Will the angular momentum of a half filled waterbottle increase? or decrease? or remain the same?
As compared to what?
when compared to the angular momentum of the bottle at instant when it started spinning.
Spinning around what axis?
about its center.
The center is not an axis. The axis is a line. It may go through the center. But there are many lines going through a point.
Look, you need all the details of a question before you can even think about an answer. You cannot expect people to read your mind and figure out by themselves what you want to ask.
Separate names with a comma.