Mystery Animal: Sea Hare?

What is this mystery animal?

I photographed this animal (if it is an animal) at Bolsa Chica Ecological Preserve near Huntington Beach in Southern California. The interpretive sign at the entrance to the preserve (I read interpretive signs religiously, of course) indicated that I might see giant sea hares near the boardwalk. Obligingly, this creature had positioned itself in that location. Perhaps the appendages extending from the front of the animal are gills?

The habitat is a muddy marshland with many species of terns and snowy egrets.

Please send me an email with any thoughts on the identification of this animal. Thank you so much for any help you can provide.