Here is a swim workout that requires a video to best explain. The focus is on three events:
Life Saving Buddy Tow — 25m Combat Swimmer Stroke 50m Freestyle 100m (6–10 strokes per breath)
The goal is to push yourself on the buddy tow — recover with the 50m CSS — then push your heart / lungs with 100m freestyle hypoxic type swim set. Check out this video for more.