The present invention relates to an evaporator of a working fluid for an OTEC plant, comprising an elongated evaporator body extending along a main axis, a bundle of evaporators transporting hot water, a sprinkling system arranged in an upper part of the evaporator body, a system for evacuating the fluid in gaseous state and a guide system for the fluid in gaseous state towards the evacuation system. The guide system comprises an elongated cover extending along the main axis, covering the bundle of evaporators and the sprinkling system, and two partitioning means which are arranged at each end of the evaporator body and form on each of these ends a sealed connection between the outer surface of the cover and the inner surface of the evaporator body.