Swim lesson help...?

You are right, parents hate seeing their kids messing around and behaving badly. When they do this sit them up on the side of the pool to remind them you are in charge and their behavior is inappropriate. To keep them interested always have them doing something even when you are working with each one individually. If you are working with a single swimmer doing arm strokes, then have the rest of them working on armstrokes on the wall until it's their turn. You can also have them do bobs, dive for rings, kick on the wall, etc. After each skill give them a reward, which could be as easy as jumping in the water, a high five, etc. Be excited about what you're doing and the kids will fall victim to your enthusiasm.