A subsea production unit 10 for subsea treatment of oil, the unit having a frame that supports an onboard multiphase separation system 22, 24 for separating gas and water from a wellstream containing oil; and an onboard water treatment system 54 for cleaning oil from water that is produced by the separation system. Optionally, the water treatment system has a gas inlet 56 communicating with the separation system and is arranged to mix the produced water with gas separated from the oil. Optionally, the water treatment system has at least one flotation unit with a water inlet for water to be cleaned of oil; a water outlet for water cleaned of oil; and a reject outlet for gas mixed with oil cleaned from the water. The water/oil separator can have least one dual pipe separator 24. There can be a pre-separator pipe section 26 upstream of the or each dual pipe separator, which can be folded sinuously to define upper and lower portions in mutually-stacked, vertically-spaced relation. There can be a sand removal unit
