Sea water flowing in the pipe. I know pressure and area of the pipe. Now, I want to know flow of sea water in the pipe. How I know?

You don't have enough information to calculate flow rate based solely on static pressure and area of the pipe. However, if you know the difference in pressure between the water at the entrance to the pipe and its exit then the velocity can be estimated from


v = \sqrt {2g \Delta h}


Where \Delta h = pressure difference expressed in terms of "head." This will give you a high end estimate of velocity, as it ignores viscosity of the water and friction of the pipe. Once you have v multiply by the area of the pipe to get flow in volume per second.