Customary international law is unstable and has been slowly eroding navigational rights -- ratifying UNCLOS will provide stability
The Convention reduces, but doesn’t wholly eliminate, the indeterminacy inherent in customary law. The Convention also provides greater stability and predictability. Here it should be noted that the LOS Convention’s articles can only be amended through an elaborate process that, by design, provides the kind of stability the U.S. has long sought in the maritime domain. By contrast, customary law rules evolve by the practice of nations asserting, acceding to or persistently objecting to new norms, thus introducing unwelcome uncertainty into the nation’s maritime affairs. Moreover, as Edwin Williamson, President George H.W. Bush’s State Department Legal Advisor noted, the history of customary international law “reflects a steady deterioration of the freedom of the seas to the detriment of the essential rights of maritime nations, such as the U.S.”
Opponents of UNCLOS claim that the United States should not become a party because the United States already enjoys the benefits of UNCLOS through customary law and, therefore, should not unnecessarily incur the treaty's burdens. However, this ignores the fact that customary law can change and can also be influenced by how parties to UNCLOS decide to interpret its provisions.