A watercraft propulsion system is disclosed for an amphibious vessels. The watercraft propulsion system includes a waterjet on each side of the vessel which consist mainly of an impeller, inlet assembly, and stator assembly. The waterjets resists vibrational fatigue and failure through specialized welding between the struts and the inside of the conduit of the inlet assembly. The waterjets can be retrofitted to existing drive shafts to meet the performance requirements of differing amphibious and other marine vessels and to accommodate space limitations. The waterjets are easily installed and removed.