Yup, they really are! Think about the mechanics of walking. You use the muscles of the hip and upper leg to move the femur (upper leg) forward. The knee joint is relaxed, allowing the lower leg and foot to trail behind. Once the knee has moved far enough forward, the momentum created from the motion causes the foot to swing forward and just as our knee joint fully straightens, we land on the foot, and our body uses that leg to support our body as it moves forward, with the hamstring and gluteus pulling the leg back behind our body. Meanwhile, the other leg has finished it’s pulling phase and is swinging forward.
Now imagine that same action, but with everthing moved from a vertical positioning of the body to a horizontal positioning. The only joint that is in a specifically different position is the foot, as the ankle allows the water’s resistance to press it back, so the toes are pointing straight back toward where we’ve just swum from. Otherwise, it’s all the same. The power is created from the upper leg and hip muscles. The knee is not rigid, but relaxed. This means that the leg does not stay absolutely straight, but rather, has some flexion and straightening through the kick cycle. It does not, however, bend dramatically, which would only result from actually using the knee as a power source for the kicking. (Imagine how silly you’d look if you purposefully bent your knee and then pressed your foot forward from the knee each time you took a step while walking.)
So give it a go. Next time you are in the pool, instead of focusing on “kicking your way forward”, just try to “take a step” in front of your body. If you keep it small and quick, you just might find it gives you better results with a lot less effort.