Side kicks is a kickboxing move that you can do in water aerobics. What you want to do is, you want to get a good stance so you're balanced. Lift up your knee, kick it out to the side, and bring it back.
If you're more comfortable with your arms out, you can do it with your arm out, or you can even hold on to the side of the pool and do it. So you lift the knee up, kick it out, bring it back in and switch.
You can work on one side at a time, or you can alternate. I would do about 12 reps on one side if I was doing one side, and then switch. Or, if you're alternating just do 24 of them. So, you lift up, kick out and alternate, kick out and alternate. You're probably going to want to kick down towards the bottom of the pool. If you want, you can lean a little more and kick a little bit higher, but that's also a little harder.
Sidekicks is really good to do in water aerobics. It's a kickboxing move that works your core, your quads, hamstrings, and inner and outer thighs.