Huh, found a bug.
Once the water meter reaches 0, you aren't able to collect more water, water the tree, or even chop down the tree.
Thank's so much for telling me! There is a restart button when your water meter gets to 0%, but I just tested it on the index.html tab and it turns out that while coding this, the restart button somehow got moved underneath the white box that contains the tree. So there is a restart button, but it is just off-screen.