Personally I have that awesome thing where you don't float. The swim instructors at TBS thought I was joking till they asked me to just lean forward exhale and float, they watched in amazement as I sank to the bottom. One thing I learned was...relax and pay attention. The instructors at API have seen every permutation of swimmer and non swimmer and will teach you techniques to get through the events.
Dunker wise, like has been said previously sit tight and let everything settle down, while holding onto your hand holds. Rushing for the exit is a great way to get a boot in the face, plus you can hold your breath for at least 2 minutes. When the rush is past you, go hand over hand to the exit and get out. I think I had the co-pilot seat for the blindfold test and did just that. A point of pride from API is I never had to redo a single event, not treading water, dunker, SWEAT for someone who spent 2 of the 5 minute water tread with his head beneath the surface.....
semper fi