Where other creatures struggle to fight and move in water, aquatic creatures excel.
Models with the Aquatic special rule 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. However, they are still subject to any special effects that specific terrain may have (e.g. we don't exempt Aquatic models from the dangers of marshes).
In addition, models with this special rule can march, claim rank bonus and be steadfast even when in water. Furthermore, if every model in a unit has the Aquatic special rule, and the majority of the unit is within water terrain, enemies shooting at that unit suffer an additional -1 To Hit penalty.