Can someone identify this rock?

Hi, while on a vacation to the island of Maui, I came upon an extremely smooth black rock floating about 300 feet from the shore of Lahaina. It floats in a normal cup of water. I figured it might be a deformed shell of some sort, but either way I cannot find another example after google searching my head off. It looks nothing like pumice or scoria, at least of the examples I’ve seen. Here’s a quick image of the rock in question.

https://ibb.co/j9Cc3pG