There are so many things to coordinate when swimming. One of the many things I like about Total Immersion (also Chi Running) is they instruct you to focus on one or at most two things at a time. So one length across the pool you focus on proper head position. Your arms and legs might be completely wrong but you don't worry about that. Coming back across the pool you pick a different focus, like body roll. Eventually it all starts to come together.

It reminds me of practicing hands alone when I was learning piano. Right hand once, left hand once, hands together.