I have added a rudimentary map of the locations at which E. diomedea has been found during our fieldwork during the past few summers.  At the moment, it provides a framework to which we can add more sightings as we turn up more slugs.  

The short summary is that suitable habitat, consisting of rocky bottom with growths of Codium macroalgae, is distributed throughout the bay, and that the ability to find Elysia can vary wildly at the same site from one day to the next.  With enough time and effort, I expect the little slugs would be found anywhere there is something to eat.