Divers and remotely operated vehicles are both integral to any underwater or subsea operation that is carried out for all the petroleum operations discussed thus far. Both carry out some common operations although some are an exclusive specialty of either of these, for example, in deep or ultradeep water ROVs can operate safely whereas in shallow water divers become more useful and cost-efficient. Sometimes diving is also supported by a fleet of ROVs in the offshore oil industry. In any case, the support from either a diver or an ROV or both is a must for working in offshore and hence the nature of work of each of these in an underwater environment needs to be understood without which the subject of ‘offshore petroleum operation’ will remain incomplete.