While I do not have personal experience with having a TKR, I am an athlete and have used swimming as a way to help in my recovery from significant sport-related injuries.
You are right to be concerned with not wanting to cause damage to your knee with the thrusting movement of a breast stroke, but at the same time, swimming is a very low-impact exercise. The bouyancy of the water will not only help support your body, keep muscles and joints loose, but also helps keep your muscles and joint from absorbing the shock they may experience when pounding the body in exercises such as running.I do not feel that the leg motion used in the breast stroke will place a significant shock to your knee. This really all depends on your comfort level. If when doing this stroke, you experience a lot of discomfort, you should listen to your body and discontinue the exercise.
If you are concerned about using the breast stroke in your swimming routine, I would suggest simply replacing it with a different stroke. Finding other low-impact exercises may also be beneficial. Some that I can suggest include cycling, using the eliptical machine, and even yoga.I have used all of these exercises to recover from fractures, sprains, knee pain, shin splints, etc.
I hope this helps you a bit!