The density of water is nominally 1,000 kg per cubic meter. However, the density of water is dependent on the temperature and pressure, and the density of water at STP is 998.2071kg/m3.
STP is standard temperature and pressure (STP) which is 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 105 Pa (100 kPa, 1 bar).