[CydeWeys]

It is used on the Big Island, which is the newest island. Unfortunately most people live on the older islands, which aren't above the volcanic hotspot anymore. And the islands are far enough apart that they all have their own independent electrical grids.

[yellowapple]

Even if they ain't directly on the hot spot, couldn't they still be close enough to it for there to be a useful temperature differential?

[gonzo]

There were attempts to use the ocean for a heat sorce/sink for downtown AC.