Now you have to keep your knees together.

I hold those things how?! In between your thighs, just above your knees. Pull-buoys. They adjust to fit between your legs, one buoy in front, the straps in the middle, and one buoy in the back.

Use pull-buoys to help your legs float while you don't kick and pull yourself through the water. It's a strength workout for your arms. It's worth it, try it. Your stroke will get better, more efficient, and stronger. It's like using swim fins, only opposite.