OK I admit it- I'm a hack in the pool. I don't warm-up, and never cool-down. I like to use the props- webbed gloves, pull-buoy, zoomer fins, etc, just to break it up, but in reality I do it to make swimming EASIER!
The classic workout for me has always been 10 x 100 meter repeats with short rest. The goal always the same- break 2 minutes. I mix in other stuff like 200s, continuous swim, etc, but always go back to the 100s. Not anymore.
In running, I'm a huge fan of 800 meter repeats, a.k.a. Yasso 800s- for me the most recently the goal was to run them under 3 minutes with one minute recovery. If I could do 10, I knew I was in pretty darn good shape.
One day in the pool it dawned on me- why not swim 150s instead of 100s- those are closer to running 800s in time and effort. Sure enough, I have been able to hit 8 x 150 m under 3 minutes, and for me that is a great workout. Last Sunday, I took the courageous step and increased in to 9, averaging about 2:56 with 45 seconds recovery, but simply did not have it in me to do just one more. I quit at 9, totally exhausted, mentally and physically.
Effort wise, the 150 m swim repeats feel about the same as running those Yasso 800s, and I think, tell me that my respiratory system is in decent shape. Now I need to finish it off and do 10. If I just had the leg work from running to go with it!