CPV code 76121000 designates Subsea well diving services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
76121000 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 76120000 (Diving services incidental to gas extraction).