How does water in general influence the climate in a region?
"Water is life"--Wallace Stegner
Water infuence the climate in many ways.
The amount of water affects storm systems. I.E., London and Seattle are ALWAYS rainy. Chicago is windy because the wind sweeps across the plains. In the winter, Chicago receives a lot of snow because of the water nearby (Seattle does as well). The amount of water does NOT effect the temperature of a region as much--although typically the closer you are to water, the cooler it is (however, Louisiana is not going to be too much cooler than states to the north not lying adjacent to the ocean.
The type of amount of water you get affects the type of plants which will grow. It also affects how thick the vegetation will be.
If you are referring to social climate, water will also affect the number of people living in a region. If the body of water is concentrated a larger group of people will move there and be concentrated (ie Chicago). However, bodies of water which are not concentrated (I.E. the Nile river, the oceans, the Mississippi) will have many cities and groups of people dispersed along the edge.