Antimicrobial resistance patterns of Escherichia coli strains isolated from 30 seawater samples and 30 Korean rockfish Sebastes schlegelii collected from fish farms off the South Coast of Korea in 2018–2019, were investigated. The isolated E. coli (154 strains) showed the highest antimicrobial resistance to tetracycline (44.8%) followed by chloramphenicol (38.3%), streptomycin (37.6%), and trimethoprim/sulfamethoxazole (35.7%). The antimicrobial resistance rate was higher in E. coli strains isolated from Korean rockfish than those from seawater. Multiple antibiotic resistance (MAR) against at least three antimicrobials (MAR index: ≥0.2) was observed in 35.6% and 79.2% of the seawater and Korean rockfish isolates, respectively. In conclusion, continuous monitoring is required to identify changes in antimicrobial resistance and their correlations in the aquatic environment and products.