Bubbles reduce drag under a ship's hull.

https://youtu.be/KF4ULDEorro?si=sUJqQuRZT5NvdRWq

Microbubbles released under a large ship's hull reduce drag and improve fuel efficiency by 20%. 

Apparently, large bubbles are better than small bubbles. 

https://www.sciencedaily.com/releases/2016/09/160902125334.htm