What techniques do we have to pour concrete under water?

The techniques for pouring concrete under water are based on the principle of getting the concrete as close to where you want it, because you don't want the concrete to mix with the water. So all the principles are more or less a pipe, in a certain shape.

Also the concrete differs form normal concrete

When building a boat ramp or a bridge you will have to learn how to pour foundation concrete underwater. The important thing to remember is that while the finished product will be underwater, the construction technically is not. Through the use of a cofferdam you can create a dry space in which to work even in the middle of a river. While constructing a cofferdam for a project such as a bridge is enormously complex and requires special engineering, the principles are the same as creating a cofferdam for a shallow water boat ramp.