Is it a constant depth pool (like most lap pools) or does it have a shallow and deep end with an angled drop (like many indoor high school pools that have swimming and diving)?
If it has an angled drop, you could mark spots for each person on the angle, with them facing the deep end for the photo. Making a choir setup, so to speak but having them sitting on the number might be easier. With a number for each spot and each person with a number, they could know their location and do a test dive, etc. The angle might provide several rows of swimmers so they could be tighter for the shot.
With you on scuba waiting, and setup, your assistant could have them all drop to their spots for the photo.
They would need a weight to hold them in place and make it easier to sink. they could just hand hold the wt.
Or having steel bars for rows sunk in place would give them a weight to grab and get the lines straight. You could have numbers painted on the bars.
Swimmers of that caliber should be able to hold their breath for longer than ave. Teams used to do breath control drills and likely still do.
Just a thought.
Here is another.
You could consider an over/under with part of the team sitting on the pool side and part of the team underwater, below them. Matching lighting might be interesting but you would have less horizontal space to light underwater. Could make it 3 rows, one sitting on the side, one standing on the bottom, one on knees underwater - held in place by those standing. It would take some timing and a few attempts.
But I have no experience with the photography, only with being the swimmer.
