If the basis of jurisdiction be a cause of action otherwise justiciable in admiralty, then, notwithstanding anything herein to the contrary, the party claiming to be aggrieved may begin his proceeding hereunder by libel and seizure of the vessel or other property of the other party according to the usual course of admiralty proceedings, and the court shall then have jurisdiction to direct the parties to proceed with the arbitration and shall retain jurisdiction to enter its decree upon the award