I'm no lawyer but I'm sure all nations can police sanctions, international waters or not. Gibraltar say it was in their (UK) territorial waters. Spain (predictably) are disputing this as they do not recognise Gibraltarian (?) territorial waters at all. https://elpais.com/elpais/2019/07/05/inenglish/1562310818_569487.html So the legal basis for this is the tanker was suspected of breaking EU sanctions and was passing through EU territorial waters.