U.S. would lose capability to interdict and hold terrorists under UNCLOS and ITLOS
Far from treating such seizures as remote hypotheticals, the Bush administration has invested considerable effort in a "Proliferation Security Initiative" (PSI) under which the United States has signed agreements with states that provide flags for most of the world's commercial shipping. These agreements may strengthen U.S. claims to intercept suspicious ships on the high seas, when flying with markings from the most common flagging states (such as Belize, Panama, and Libya, which have all signed such agreements). But the PSI agreements do not make clear when or whether ships or crews may be subject to long-term detention, and all the agreements stipulate that they do not supersede accepted standards of international law.
If we ratify the Law of the Sea treaty, even a PSI agreement with the flag state won't necessarily keep a dispute about the seizure from winding up before the Law of the Sea tribunal in Hamburg. That tribunal has asserted its right to hear claims for "prompt release" when filed by owners or operators of a ship, even when the nominal flag state takes no role in the proceedings. In past cases, ITLOS has ruled that ships cannot be detained, even when claimants refuse to supply full information about how the ship was acquired and on whose behalf. So while we have jealously reserved the right to detain terror suspects captured on land, we will, if we ratify this treaty, give up our right to decide when we can hold terror suspects seized at sea.