Ah, good question! I forgot to mention this explicitly anywhere.

Tiles are connected by horizontal and vertical adjacencies (no diagonals). Amphibious adjacency works by calculating horizontal+vertical paths through water tiles to other land tiles.

But this doesn't really work well for long distances. So I've limited the lengths of these naval paths to ~10 tiles.

The Island of eternal peace is more than ~10 tiles from any other landmass, thus it is completely isolated.

At some point I want to add in a system that allows contact across larger distances. But note that at this resolution, a distance of 10 tiles ≈ 1,000km. Which is more than the distance between almost all islands and island-continents on Earth and their respective mainlands. And I do want the sim to allow for situations such as the Eurasia<->Americas divide where you've got de facto unconnected civilizations evolving independently in different parts of the map.