In my opinion the Combat Side Stroke is by far the most efficient style of swimming. Fun fact I can also swim just as fast with the side

Side Stroke Swimming: All You Need To Know

What muscle groups does swimming develop and its benefits - ActiveSG

Swimming: How to Swim a Side Stroke

Side Stroke Swimming: How to & Why Navy SEALs use it

The Side Stroke – Judy Sunde Hanawalt

Swimming: Freestyle - Physiopedia

Learn 8 different swimming strokes and styles – Ninja Shark Australia

Types of Swimming: Learn Various Swimming Strokes And Styles

The Pros and Cons of Popular Swimming Strokes

Combat swimmer stroke (CSS) vs Elementary Side Stroke. Both are great for the Navy PST.