On 16 September 2021, the leaders of Australia, the United Kingdom (UK), and the United States (US), announced the AUKUS arrangement, a security agreement which provides a framework for nuclear-powered submarine technology to be shared by the US and the UK with Australia, as well as other forms of new military technology. This article explores the legal im-plications of this alliance and the increase in technological sharing and maritime cooperation it promises. It first provides an overview of the AUKUS arrangement, situating it within the long history of cooperation be-tween these three countries. It then considers how international law regu-lates the sharing of maritime military technology, contrasting the significant constraints on sharing nuclear technology with the minimal restrictions on sharing non-nuclear technology. Lastly, the legal implications of States working closely together on the ocean are addressed. While there might be military and strategic advantages to this close collaboration, when and how States might be complicit in the international wrongs of partner States and the challenge that can be posed by legal interoperability are explored.