While cruising, there are so many things in nature that you just don't anticipate. That's why I always carry a couple cameras. Some events are so fast you never get an opportunity to take that once in a lifetime photo, but then, sometimes you are fortunate enough to capture those images, both video and stills.
I walked out on the dock tonight where my next door neighbor has suspended an underwater lamp. The things that I've seem have been fascinating to say the least, but none as fascinating at this large, swimming nudibranch.