The sea is dark until you see it; once seen, it stays on your chart.
1A new captain's chart is mostly fog: only the start port (6 km around) and what she has sailed past. Unknown ports show as dim signs without a name.
2Your ship sees 2.4 km around (less in rain and fog, at least 1.5 km); every port visited opens 3.5 km. Lookouts and buoys chart more.
3Your island sees 4 km plus 1 km per Town Hall level; a lighthouse and a signal tower add 1.5 km each, a chart house 2 km. Up to 3 scouts sail out to 30 km.