If you want to get a good workout for your legs in water aerobics you can do front and back kicks. They're very easy to do and what you would do is you would just step forward, kick your leg front, step back, kick your leg back.
You can put your hand on your hips to make it a little easier or you can hold they out to the side if it's easier for you to balance. Another way to do front and back kicks is to use one leg. So, you can kick it forward and then using the same leg you kick it back. So, kick it forward and kick it back. Make sure when you kick, you don't want to lock out your knee because that could hurt. Kick it forward, kick it back. You can do it for about two minutes and then one minute on each side. Left leg, right leg one minute and that would be actually a good warm-up, front and back kicks. I would use that as a good warm-up set. It works you quads, your hamstrings, and also your core because you're balancing, so it's great for your balance too.
So, front and back kicks, you can mix it up. Forward and back or use the same leg for about a minute on each side.