I prefer PFDs with a flat back for dragon boat (ie the Lotus "Lola" for women -
http://www.paddling.net/Reviews/showReviews.html?prod=522), if you get someone in front not reaching as far as they could be with a bulky PFD on it just gets awkward. Even if they are reaching and you have a long reach, it's easy to get hung up on that little PFD ledge with your knuckles with some of the lower profile PFDs.
As for the inflatable PFDs, they may be approved but when I was with Coast Guard Auxiliary we never recommended them as if they malfunction (hole in flotation, gimpy canister) and you happen to be unconscious after whatever landed you in the water, you're pretty much out of luck.