You can swim up in water that is enclosed in cactus if you go to the edge of one of the sides of it - collision glitch

You can swim up in water that is enclosed in cactus, therefore it has to do with collision.

Reproduction
1. Make cactus hole by enclosing cactus.
2. Pour water inside cactus
3. Go to the edge of one of the cactus and you will be affected by water.