You need to get a book called Total Immersion.
The essential difference between swimming and most other sports in the level of drag created by water as opposed to air. Because water creates so much drag, increases in “power” are very easily negated by poor form. If you can get used to a body position which minimises drag you will drastically reduce the amount of power required.
I swum a lot as a kid and went back to it a couple of years ago. I was doing 2000m in about 45 minutes, getting out of the pool hyperventilating and slightly cross eyed. I can now do 2000m in anywhere from 33 to 40 minutes, after a gym session, and get out of the pool feeling more relaxed than when I got in.
If you’re swimming for triathlon there is a big advantage to maximising your efficiency, especially if you can get on somebody’s toes as you get a great draft off other swimmers.