The primary concern of fiber-reinforced polymers (FRPs) subjected to seawater environment is losing their initial mechanical performance since water can diffuse into the composite and deteriorates the fiber-matrix interface. Recent studies related to aging performance in the seawater environment have shown that introducing halloysite nanotubes (HNTs) into the polymer matrix offers a combination of an efficient barrier effect and an improved fiber-matrix interface. Hereupon, the principal objective of this study was to experimentally investigate the impact of HNTs on shear and mode II fracture performances of the seawater aged basalt fiber (BF) reinforced epoxy (EP) composites. After six months of aging in seawater, the findings indicated that HNTs reinforced multi-scale composites exhibited 34 and 46% higher shear strength and mode II delamination toughness compared to the neat specimens. Moreover, according to the dynamic-mechanical analysis, higher glass transition temperatures (8%) were obtained for the multi-scale composites. The reduction in mechanical performances induced by fiber-matrix interfacial degradation was also confirmed by scanning electron microscopy analysis. Chemical deterioration of the polymer matrix was explored by Raman spectroscopy to reveal the efficiency of HNTs induced barrier effect. As a result of these investigations, HNT modified BF/EP multi-scale composites were offered for future advanced engineering applications.