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Bacterial ability to deploy the type VI secretion system (T6SS) against rivals requires prolonged cell-cell interactions. Such interactions are facilitated on solid surfaces but are assumed to be absent in liquid, leading to the conventional dismissal of T6SS-mediated competition in liquid environments. Here, we find that Aeromonas jandaei employs its T6SS to eliminate diverse bacterial competitors in liquid media. Using a workflow that monitors interbacterial competition via prey luminescence, we demonstrate that auto-aggregation and co-aggregation, facilitated by distinct adhesins, enable kin and non-kin recognition and intoxication in a T6SS-dependent manner. Furthermore, we show that another marine bacterium, Vibrio coralliilyticus , employs T6SS to intoxicate rivals in liquid media. Collectively, our results indicate that T6SS-mediated competition in liquid is more common in marine bacteria than previously anticipated, and can be facilitated by diverse molecular mechanisms that govern cell aggregation.