Is Lloyd's Open Form the only form of salvage agreement used at sea?

No, but it is by far the most popular. Various other forms are in use around the world, some of them mandatory in a coastal State's waters. In Japanese waters, for example, the Japanese Form of Salvage Agreement may be offered by a local salvor. The U.S. Open Form Salvage Agreement may be used in the US. There is also a Beijing Form, a Moscow Form, a Hamburg Form and others.