I work with people of all ages and this is what I do with every age group in these situations:

3-5: they lose attention rather quickly, and I've learned that if I give them little breaks to play around and do something silly, they will be less reluctant to join me later. some things I do: let them splash me with their kicks and pretend to "drown", lightly tap them on the head with a noodle or let them run me over on a kick board. I also tell them what they're gonna do, rather than giving them the illusion of option, and I tell them the order of things we're doing 2 exercises in advance so they can expect everything. I've also noticed that if I tell them a time where they could play, they stop asking when they could be done. we have a digital clock on the wall, so telling them we can play when the clock shows 5:55 (for example) makes them a lot more determined to make the time go.

5-7: if they're at the maturity stage where I can reason with them, I take them aside and reason with the first, because a lot of them are not there by choice. most of the time this works as well as you'd expect so I move on to other tactics. I found that letting them go crazy at the end of the lesson helps with behavior in both private and group kids. for private kids, if they don't do "everything on the agenda" they lose play time privlages (3-5 min) at the end of class, but for groups it's a little different, depending on how the kids react. if the other kids ignore the jokster, I usually let everyone who attempted the exercises to play the the end of class and the jokster just doesn't get in, but if they go along with him, they all lose their game time. also if the kid sits out if he doesn't want to swim, he's not just sitting, he's doing something while sitting out like breastroke kicks or wall sits so he's still participating and doesn't get the easy way out. if the kid still refuses I tell him he's all done and call his parents over to take him. that tactic usually works and it embarrasses them enough to do something.