major_rivers: All sea zones listed here are major rivers, and can be navigated by any religion with allow_rivermovement = yes.

ocean_region: This contains a list of sea zones (automatically numbered based on their position in the file), all of which can be navigated to from the others.

(Exception: A sea province that has no neighboring sea provinces is assumed by the game to be a lake, and does not generate ports adjacent to it.) Note: Each sea province in a sea zone is considered to be reachable by ship from every other one in the same sea zone. You may have multiple sea_zones defined, one for each range of contiguous sea provinces. You must have exactly two province IDs listed for each sea zone.

sea_zones: This contains two sea province IDs that are at opposite ends of a range of contiguous sea province IDs.

If you forget to add one, the last province will get ignored.

max_provinces: This is the number of the last province + 1.

Here are the ones you need to worry about:

Default.map contains general information about the map.

Aoe 2 custom maps