this may sound like a dumb question, but do you have short legs and a long torso? At the comprise of my humility, I'm pretty good in the water, strap a kickboard on me and I don't go anywhere for more than 25 meters and I'm spent. Keep doing the f...
I'd say do your best bet would be to get enough air in the lungs right before you make the descent you think you're going to suck at, and as you exhale, look up, it will increase your rate of descent