Here's a pic from a while back. It's of a Horned Helmet Snail (Cassis cornuta). This is one of the larger shells found in Hawaii. Resembling a Conquistador's helmet, it can grow to a bit larger than a foot. Many people who've been to the Caribbean will refer to it as a "conch" although it's not... different genus of shells, Hawaiian conchs are pretty small.
We often find these on sandy bottoms, many times mostly buried. They feed off of sea urchins... this one's pouncing (it "pounces" sooo faaast, if you blink, you'll have time to blink several more times, before it finishes the "pounce" - they're generally pretty slow) on it's prey in this picture.
It was quite gray today in Kona and much of the state, the sun almost broke through early on, but we basically were under clouds most of the day. I guess we're expecting more of the same the next few days.