Sea Creatures can move within any area of water on the battlefield, including rivers and even deep water that players may have deemed impassable to other models, as if it were open ground (see Battlefield Terrain for more on terrain types). However, when out of the water they cannot march. Sea Creatures are still subject to any special effects that specific terrain may have (e.g. we don't exempt Sea Creatures from the dangers of marshes). Being a denizen of the deep seas does not necessarily equate to protection from a choking quagmire.