You can do this by animating a Mask Path.
Make a Solid of blue color and then create a shape of water spill using Pen Tool. And then animate the Mask Path parameter to the desired shape.
Go through these links:
https://helpx.adobe.com/after-effects/using/animating-shape-paths-masks.html
If the morphing is between complex shapes then you can go for Reshape effect.
See the tutorial:
https://library.creativecow.net/articles/zwar_chris/morph.php
Vishesh Arora
3D and Motion Graphics Artist
Films Rajendra
Blog:
https://digieffects.wordpress.com
2011 3D Demo Reel:
https://www.youtube.com/watch?v=lHPgIJU_BR8