Bodies of water, regarded in the terrain editor as Ocean, are, alongside land, the most important type of terrain, as it determines the game map itself. Any map is defined first by how much land it has in relation to water, and vice versa. In addition, the presence of bodies of water in a map provides a second, entirely different strategical teathre compared to maps without them. For instance, Ocean allows for Fish and Whales, Rare Resources which only exist in ocean, and can only be gathered by Fishermen, to be present. When the appropriate technology is researched, units can automatically take Transport Ships to move through Ocean to trade, explore, fight, and even colonize new continents far beyond the nation's borders, allowing for greater tactical flexibility and faster expansion than could be possible in land maps. Ocean is also a source of Oil patches in abundance.
All these advantages also ensure that competition for control over them is always a possibility, and for defense of the waters, a Navy may be required. This may bring additional strain to a nation's economy, although in many maps, just as in world history, whoever achieves control over the waters can gain a decisive advantage, which can make the difference between a secure victory and a hasty defeat. The benefits of gathering from the water's riches may also surpass the costs of protecting them. As a general rule, the more water a map contains, and the richer it is in relation to the land around it, the more necessary it is to aim for naval supremacy.