I just stumbled on this thread. I know it's a good few years old, but feel I have to reply for the benefit of anyone else asking the same question...
So - the original question was why do tense muscles make you sink, or don't they?
Answer - they don't. It's a complete fallacy. (Like so many myths in swimming).
Moral - don't believe everything you're told or read about swimming (which I would hope TI practitioners wouldn't!)