Hey there, new to the forum. I did API about 2 years ago, so take this for what's it's worth.
The swimming is all mental. I suck at swimming, and I had no problems.
The longest I ever had to hold my breath was about 30 seconds, and that's just because of some numbnuts in front of me in the dunker. Watching people do the dunker before me was pretty scary, but once I did it I thought it was a blast. That was my favorite part of water survival.
The prone float sucked, but that's just because your heart rate is high from treading water.
About the only time the instructors were picky on technique was for the strokes test, and even that was pretty easy.
As other people have said, the key is to pay attention to the instructors, stay calm, and have fun. API is the easiest part of flight training, so don't worry too much. It's more an issue of not know what to expect. But just remember that if I can do it, so can anyone reading this post.