NOVELTY - The subsea unit (1-A) has a two-phase cooling system (2) that includes an evaporator (4), a condenser (5), and a conduit (3). The conduit forms a closed loop to circulate a cooling fluid (6-1) together with evaporator and condenser. An enclosure (1-1) is arranged to enclose the cooling system and provided with a portion that is mechanically flexible, such that an ambient subsea pressure is transmitted to inside of enclosure. A liquid (L) is subjected to an ambient subsea pressure higher than a pressure withstand by the enclosure, such that deformation of enclosure is counteracted.    USE - Subsea unit for housing electric device and electronic device such as insulated gate bipolar transistors and integrated gate-commutated thyristors, frequency converter and transformer, of subsea power system such as transmission system and distribution system (all claimed), for use in oil and gas industry, subsea high-voltage, direct current (HVDC)/ heating, ventilation, and air conditioning (HVAC) transmission and distribution systems and for offshore power generation of wind energy, tidal energy, wave energy, and ocean current energy.    ADVANTAGE - The liquid fills the interior space of the subsea unit so as to prevent the occurrence of any air gaps between the inside surface of the enclosure and internal component of the subsea unit and to prevent short circuit of the electric or electronic device. The simple passive mechanical valve construction is provided, such that the risk of equipment failure can be reduced. The significant overpressure in the conduit is avoided at any time, such that cooling fluid is environmentally friendly in nature to avoid harmful effects of the cooling fluid to the environment.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a subsea power system.    DESCRIPTION OF DRAWING(S) - The drawing shows a side view of the subsea unit for subsea installation.    Enclosure (1-1)    Subsea unit (1-A)    Two-phase cooling system (2)    Conduit (3)    Evaporator (4)    Condenser (5)    Cooling fluid (6-1)    Liquid (L)