Not offered as 'gospel', my understanding is that the "minimum 500 foot" rule would be the key, interpreted as being the required separation from living creature (man/animal) or man-made object ..... not trees or land as such, not birds (presumably), not other aircraft (formation flying?), and not when landing or taking off in accordance with normal aviation practice.
Thus for example 10 foot agl could be legal, providing you can guarantee no person / animal / man-made object within a 500 foot radius. Not easy to guarantee.
Similarly 10 foot above the sea could also be legal, providing lateral separation from yachts, ferries etc is at least 500 foot.
Lack of any safety margin is of course a separate issue....