Marine infrastructures play an important role in the social-economic development of coastal cities. However, the shortage of river/manufactured sand and fresh water is a major challenge for producing concrete on site, as the transportation of these materials is not only costly but also environmentally unfriendly, while desalination of sea-sand and seawater is also pricey. Seawater sea-sand Engineered Cementitious Composites (SS-ECC) have a great potential for marine/coastal applications; but the present knowledge on SS-ECC is extremely limited. This study aims to explore the feasibility of producing high-strength SS-ECC. The effects of key composition parameters including the length of polyethylene (PE) fibers (6 mm, 12 mm, and 18 mm) and the maximum size of sea-sand (1.18 mm, 2.36 mm, and 4.75 mm) on the mechanical performance of SS-ECC were investigated. SS-ECC with compressive strength over 130 MPa, tensile strength over 8 MPa and ultimate tensile strain about 5% were achieved. Test results also showed that the tensile strain capacity increased with increasing fiber length, while sea-sand size had limited effects on the tensile performance of SS-ECC. The findings provide insights into the future design and applications of ECC in marine infrastructures for improving safety, sustainability, and reliability.