question: is water a whole new field, or is it considered "land" when it comes to units? simply saying, can my archers (or any land unit for that matter) attack ships without needing a whole new attack field?
It's three new fields, but 'fields' have a more refined meaning in GAE now. They are only used for movement, and they each correspond to a 'zone', in the case of land/water units the zone is the same, 'surface'. What were attack fields are now zones, but the old names and values are still used in the XML, 'land' in an attack field means the surface zone.
So, your archers will automatically be able to attack ships, yes.