Severe exploitation of vulnerable persons is occurring in fishing industries globally. An overview of exploitation in New Zealand and Thailand highlights the incentive for states to downplay exploitation and underpins the appeal of a right of visit, which is provided for under the Law of the Sea Convention in regards to the slave trade. Although reported as forced labour, debt bondage, or human trafficking, an examination of international jurisprudence reveals that current practices likely amount to slavery; primarily due to the inherent vulnerability of persons at sea. Two persuasive cases, Kunarac and Tang, provide guidance on interpreting the definition of slavery, particularly the powers attaching to the rights of ownership. The operation of a right of visit is considered against the law of the sea regime, as are the implications in the light of international attempts to control IUU fishing.