You might try a swim clinic or swim lessons. Sometimes it's helpful to have someone else give you pointers on what you're doing wrong, even if you think you're a good swimmer. Some clinics will film you under water and on top of the water to look at your rotation, stroke, recovery, and point out some drills that can help you "fix" what you're doing wrong.

If you aren't doing drills, joining a master's swim program or starting on some swimming drills on your own might help, too. Just like biking or running, varying your distances, doing sprints, and doing "form" drills can really help.