I learned to swim by reading a book in 2010. I can run a 2:50 marathon and have a half iron bike PR of around 2:10, but my best 1000yd in the pool was at a 1:44/100 pace. So I had plenty of fitness but my technique obviously sucked. In 2012 I paid for five one-hour private lessons with a legit coach. The lessons were in an endless pool with multi-angle video, mirrors etc. I didn't do much swimming in the sessions, it was hardcore instruction. After only four lessons lessons I had to go on a bunch of business trips so never did the final lesson. I went back to swimming and applied as best I could what I had learned. After a few weeks I did a 1000yd TT and it was at 1:32/100 pace. Literally no fitness improvement whatsoever, just four private lessons, and I got 12 sec/100. Several months later as I ramped up training I got that down to 1:30/100 once my swim fitness closer to in-season. I know that with more instruction I could do a LOT better. But honestly if you're looking for very quick improvement, the private lessons for me revealed a LOT of low-hanging-fruit in my technique.