What is the relationship between water temperature and its density

The first answerer is right, but I'll give you some more details.

The hotter the water, the less dense. But why? Density is the amount of water molecules in a given volume, for example 1 cm3. You know how the molecules of fluids are always moving, bumping into each other, well the hotter the water, the more energy there is in its molecules the faster they will move and the more frequent will they bump, this causes to be less molecules in a give volume.

The colder the water, the slower will these molecules move, so there will be more molecules of water in a given volume. And of course really cold water(ice) is more dense, because it's molecules don't move at all!

The density of water is approximately one gram per cubic centimeter. More precisely, it is dependent on its temperature, but the relation is not linear and is unimodal rather than monotonic (see right-hand table). When cooled from room temperature liquid water becomes increasingly dense, just like other substances. But at approximately 4 °C (39 °F), pure water reaches its maximum density. As it is cooled further, it expands to become less dense. This unusual negative thermal expansion is attributed to strong, orientation-dependent, intermolecular interactions and is also observed in molten silica.

As temperatures rise, the molecules involved move away from each other...therefore the density is lower. When temp goes down, density is higher.
I always think of it as being like a human. When it's cold out we tend to get closer and when it's hot out we don't like being close to others. And seeing as though density is a measure of how many things are in a certain area, there is your answer.

The colder the water, the more dense. Really cold water becomes dense ice. Really warm water becomes steam.