Both times we have noticed this strange looking creature it was discovered swimming in the water column, laterally flexing back and forth and looking rather ragged and forlorn (ok, serious anthropomorphism there. . . ). They were likely dislodged from their home in Halimeda kanaloana beds near by, but just guessing there.  Cory Pittman, on the Sea Slugs of Hawaii site, describes swimming  through clouds of Melibe engeli above Halimed kanaloana beds, suggesting that they have some extremely prolific years.   I’ve seen this with the Melibe leonina in British Columbia as well — entire eelgrass beds covered in them and their eggs and then nothing the next year!  Since the engeli tend to nestle among algae that generally  lies deeper than many snorkelers go, tank-free, short-breath-holding humans are likely limited to seeing them at loose ends in shallower rocky areas, or in the water column.  The individual photographed in the algae was no more than 18 mm and the one documented while swimming was less than 2 inches (50 mm), but they have been said to grow to 4″ or 100mm.  Check out some of the photos on Cory and Pauline’s site — they can indeed look quite beautiful, complete with white specks covering their rather strange cerata and blue highlights.