Status Updates From Past the Last Island

There is a scene where the islanders develop a catamaran and experiment with using it.
There are also divination practices that foreshadow Native American ones but also have parallels in the Pacific, like a medicine bundle, a sweat lodge, and divination using smoke and a turtle shell. It's all very believable.

This is my kind of book. Lots of world building, but also lots of action. Seems really well-researched too.