Im wondering why you keep talking about sinking to the bottom?
1. This is a float/ tread and since you cant float you need to be treading. This is tiring i understand but this is not supposed to be an easy skill. I dont float very well so treading is how I got through it.
2. This skill is to be done in a swim suit only so practicing in anything that will help give you buoyancy is not helping you prepare for the skill, and anyone that allows you to use anything other than a swim suit is more concerned about you passing than giving you the challenge that comes with being a prepared divemaster.