blue sea slug

I'm starting to think StirredMocha and Shanna are right with the ID... as SM said maybe a juvenile.

What a beautiful animal. It doesn't look like it's in the water - how was it photographed?

Great find, they are hard to spot. I think it's a Glaucus atlanticus - it has the long tail, which G. marginatus doesn't have. Some examples - here is a G. marginatus I spotted (it has the old scientific name on it - I will update): http://www.projectnoah.org/spottings/17220272 and here is a G. atlanticus that Scott Harte spotted: http://www.projectnoah.org/spottings/17357026

Your spotting has been nominated for the Spotting of the Week. The winner will be chosen by the Project Noah Rangers based on a combination of factors including: uniqueness of the shot, status of the organism (for example, rare or endangered), quality of the information provided in the habitat and description sections. There is a subjective element, of course; the spotting with the highest number of Ranger votes is chosen. Congratulations on being nominated!

As this is a mollusk and not a fish, I have moved this to the Other category for you.

great spotting.....

I would put "Glaucus sp." into the scientific name field at this stage. Wonderful find. Careful if handling this as they can hit you with Man'O'War barbs.

"G marginatus differs from G atlanticus in bearing four pairs of clusters of papillae that are arranged in more than one row (multiseriate) and may be 139 or more in number"..... Anyone want to count them? :-)

might be ... I'll check the trickier bits

Looking through internet pics, I found G. atlanticus juveniles to have a solid dark line. I originally though marginatus just the general shape seems to be more elongated rather than squat and the shape of the dorsal line has that "hourglass" shape at the head whereas marginatus is more of a solid black line.

Although G atlanticus is found in your part of the world G marginatus is the one with the solid dark dorsal line (among other differences)?