I'm not sure if they're the same - we have tons here in Nor Cal too, but I haven't tried eating them. I did try some on a trip to Oregon & they tasted horrible so I've been skeptical.
Maybe I'll pick a few & try them next time I go out.
Similar to murdaINC:
Generally I eat the smaller ones whole(without shell of course) with a little chili pepper water(chili pepper, salt, water, sometimes a touch of vinegar). The bigger ones, I pop them out, scrape of the poop sack, then grill them in the shell with some variation of a soy sauce/ chili pepper mix. Still have places in Oahu that have them, but you won't hear where from me
Generally in Hawaii there are 2 types - the yellow & green/gray footed opihi. The yellow-footed ones(the yellow part sticks to the rocks) are the ones sought after.
Another real easy way to test them out is to fry some butter & garlic in a small pan & throw a few on there with the meat side down. Like murda said, when it's ready the meat will easily separate from the shell. Take off the poop chunk in the back(it's not bad, we eat it on all the smaller ones) & let us know how it went. A lot easier than firing up a grill to test them out!