That'll be not quite 'out back' enough then?
Happens to everyone occasionally
(or quite often to some people, myself included)
Is it possible to duck dive a 7'10 mini-mal?
Well, technically it is, but unless you can completely submerge the board (try this in flat water!) then you'll really struggle
I know I cant properly duck my 7'4 and that'll be a lot easier than your mal.
Also, from your description Im guessing you were trying to duck dive when you weren't really moving? If so, yeah, you'll drown
You need, particularly on a big board, some momentum towards the wave as you duck underneath it.