Dense wavelength division multiplexing (DWDM) is an optical technology that allows transmitting across a fiber many wavelengths, which can be added and dropped by means of passive optical components. We present and discuss the performance of a complex DWDM network data transmission system that will find an application in the NEMO underwater neutrino telescope. The tests cover the qualification of a complete multipoint DWDM network consisting of transponders, fibers, passive optical filters, and an optical amplifier. The behavior of the network is evaluated in the final arrangement by means of bit error ratio (BER) and optical signal-to-noise ratio measurements at 800 Mb/s rate. In order to test the network, a board, developed for the NEMO experiment, has been used. A custom DWDM module, capable of data rates up to 1.4 Gb/s and specifically designed for real-time data acquisition systems, can be hosted by the board. A dedicated application runs on the board programmable logic controlling the module, monitoring the transceiver's functionalities and running the BER test.