Eggs or ???

Found this in 40 feet of water in Hawaii (Kauai). About the size of a quarter. Thinking it's some sort of egg casing?  Any ideas?

Just a guess but based only on shape I suspect a welk egg case. Below are a few pictures with a similar shape taken in Florida although they were much larger than yours so maybe from a much smaller species of welk.

Interesting, thank you for sharing!  I've never seen one of those here, but maybe it could be!

We can probably rule out Triton. https://www.alamy.com/triton-charonia-rubicunda-laying-eggs-galicia-spain-image60054786.html