AlSi9Mg silumins are increasingly used in lightweight ship structures and onboard equipment, where resistance to impact and vibration is crucial. This study evaluates the effect of AlSr10 (0.15 and 0.30% Sr) and AlB4 (0.20% B) additions on the microstructure, impact strength and fracture behavior of the AlSi9Mg alloy. The highest improvement in impact strength—from 4.8 to 11.1 J/cm²—is achieved with 0.30% AlSr10. The lamellar eutectic β(Si) transformed into a fine fibrous morphology, reducing interphase spacing and producing a more tortuous fracture profile, which enhanced dynamic load resistance. While the AlB4 addition shortened α(Al) dendrites, it did not significantly improve impact strength (4.13 J/cm²). The results demonstrate that a properly selected Sr level enables the design of components (e.g., brackets, housings, and guards) for marine environments subjected to impact and vibration. This work provides a basis for future investigations into long-term durability of modified silumins under corrosive marine conditions and combined impact–vibration loads specific to waterborne transport.