The demand for fast techniques in the control of environmental pollution is rising at an ever-increasing rate. Flow injection analysis (FIA) can provide a simple, handy, and versatile tool to determine metal cations in seawater. Furthermore, the use of FIA techniques coupled with on-line sample preconcentration and matrix modification has recently attracted considerable research effort in attempts to control interference effects. Since the analytes reported in this paper are metal cations, the most common active agents used for sample preconcentration are chelating resins of different types. The combination of FIA with detection techniques, such as atomic spectroscopy (AS), for the determination of cationic species in the general context of seawater analysis is reviewed.