I am looking for a flowmeter for marine (underwater) pipeline. The flowmeter needs to be installed temperately on the running pipe line end for measuring the current discharge flow rate. Pipe dia is 500 mm, and water depth is 15 meter. The meter must be very reliable and maintenance free.
That sounds like an application for a propeller meter. Basically something that looks like a boat propeller driving a pulse output.
They also come with a direct readout mechanical unit.
Try googling "Propeller Meter." You should be able to get a saddle type and make a frame to slip inside or around your pipe.