Our most common nudibranch (well at least I think it is) is
Ceratosoma brevicaudatum and I still have no idea if it has a common name.
It's big as far as nudibranchs go. It is bright orange which makes it visible from meters away. Yet still many divers swim right on by.
I will confess that it was more than 20 years before I saw my first nudibranch. Quite simply, if it wasn't swimming, I didn't see it.
Underwater photography has completely changed the way I dive and the things I see diving. I'd recommend taking a camera underwater to all new divers once they have gained some water confidence.
Nudibranachs are just one of the many critters that have brought the little things underwater to life for me.
Photo: Robert Rath, ‘The Little Things', 1/160s f8 ISO100 100mm