Jet propulsion is a common locomotion strategy in nature. We developed an underwater particle image velocimetry (PIV) system to investigate the hydrodynamic effects of the SALP (Salp-inspired Approach to Low-energy Propulsion) robot, a soft underwater robot that swims using jet propulsion. Multiple SALP units can be physically connected to form a multi-SALP system, coordinating their jets to achieve different motions. This thesis explores jet interactions in the multi-SALP robot under various physical arrangements and propulsion modes. PIV analysis identified three types of interactions: interaction, weak interaction, and no interaction. Results show that, compared to non-interacting jets, interacting jets can exhibit up to a 25% reduction in average velocity. This research enables further exploration of flow-field-enhanced performance in multi-SALP systems.