K4 is a de novo peptide with antibacterial activity on human pathogens. It has a short sequence (14 amino acids), with a cationic N-terminal moiety and an amphipathic ɑ-helix structure. The present paper demonstrates its activity on Vibrio bacteria in a marine environment. It was found non-toxic on marine organisms including Artemia salina, Dicentrarchus labrax, and Magallana gigas at different developmental stages, but influenced the growth of unicellular organisms like microalgae, depending on the algal strain and on K4 concentration. Furthermore, an original approach coupling liquid chromatography (RP-HPLC) and mass spectrometry (MS/MS) allowed us to monitor the degradation time course of the peptide for the first time in conditions close to a hatchery environment, i.e., in the presence of oyster spat. We detected truncated forms over time, and the full K4 was gradually no longer found in these filter-feeder oysters. Finally, using an automated optical density meter, we monitored the growth of several aquatic bacteria identified as pathogenic on animals. K4 had a bactericidal effect on Aeromonas salmonicida and Vibrio splendidus LGP32 at concentrations below 45 μg mL −1. Our results show that K4 could be an environment-friendly alternative to antibiotics, non-toxic to several marine organisms. The use of K4 would be particularly useful to decrease the bacterial load associated with food intake in the early developmental stages of marine animals reared in hatcheries.