What to do, as nudibranchs have no faces?
When you take photos of people or pets, you focus on their faces. If the eyes are in focus, the photo generally looks great.
Now, nudibranchs don’t have eyes, but they are pretty photogenic (if you’re into critters that have weird li’l horns and tail tufts like a riverine rabbit )
And therein lies your answer
Where possible, get the rhinophores — those ‘horns’ — in focus. And if the nudi won’t help out and look you in the eye… erm, won’t point its rhinophores in your direction… you could focus on the tail tuft (which is actually its lungs) instead.
When photographing nudis, aim to get the horns or the tail tuft in focus