Wouldn't it be great if learning a new skill was predictable - each week we made improvements, each week we developed a new skill and each week we felt that sense of accomplishment that comes from knowing you're getting better.
Learning to swim is definitely not a linear process - as hard as it is for a swimming teacher to admit! If I wanted to really market my swim school, I'd promise that all my swimmers would make steady improvement week by week, and some certainly do. But often the younger babies and children and adult beginners face a learning curve that's a little less neat and tidy.
All sorts of things can affect how we develop each lesson. For a baby, maybe they're hungry, tired, teething, unwell or going through a particular developmental stage. For an adult, maybe you have something on your mind, maybe you have an injury or strain or maybe you're just feeling tense (which will affect your buoyancy and balance in the water and your ability to concentrate). Maybe something happened to knock the swimmer's confidence the previous week and they need to regain their faith in their own ability or their trust in whoever is bringing them to the pool. An adult or older child could find a certain stroke much more challenging and seem to take a step backwards when it's practised or introduced.
I absolutely don't agree with or support any idea of the "Water Wobbles" - any child who is working at a pace right for them won't suddenly be scared of the water. In my experience that comes from the conditioning often used by swim schools when an adult submerges the child under the water - as long as children are given freedom in the water and able to build trust they won't be develop a fear.
All children and adults absolutely do progress when you look at their swimming over a period of time, but don't be put off or disheartened if the swimmer can't seem to do something that seemed easy the week before, and certainly don't force a baby or child to do something you feel they ought to be able to do - they will quickly pick up on your frustration and it will eat into their confidence.
Go into your swimming lessons with an open mind - you might be pleasantly surprised one week and mildly disappointed the next, but you can't lose when you're working at the pace of the swimmer.