The map has lakes and inland seas. Water tiles can't be crossed or settled; troops and merchants travelling overland go around the water, so the land route to the far shore is long.
Villages next to water are coastal, and only coastal villages can build a harbour. From a village with a harbour you can send troops by ship in a straight line to any shore on the same water; when sending, you see both the land and sea times and choose. The number of troops per voyage and the ship speed grow with the harbour level; troops that sail out sail back. Merchants use the sea route automatically when it is faster.