Long and short passing practice.

Depending on the passingskills you can lengthen or shorten the distance.

Or add the number off passes while the middleman is swimming.

2 passes to 3.
4 swims towards 3.

3 passes to 4.

4 passes back to 3.

4 takes the place of 3.

3 passes to 2 and start swimming towards 2.

2 passes to 4 and 4 passes back to 2.