In the present study, the flow field of a momentum jet discharged vertically into a wave environment has been investigated. Three distinct processes which increase the dilution rate of the pollutant concentration have been identified by the flow visualization method. They are jet deflection, “wave tractive mechanism”, and wake vortex structure. A highly precise LIF-LDV system was developed to measure simultaneously the two-dimensional velocity field and tracer concentration. The measurements show dramatic variations not only in the longitudinal distribution of the mean jet axis but also in the cross-sectional profiles. A realistic model is also derived to estimate the initial dilution rate under the wave action.