A fast and sensitive ion chromatographic method for the simultaneous determinations of six inorganic anions [iodate (IO3-), bromate (BrO3-), bromide (Br-), nitrite (NO2-), nitrate (NO3-), and iodide (I-)] in seawater is described using dodecylammonium (DA+)-coated monolithic octadecylsilyl (ODS) columns, a concentrated aqueous NaCl mobile phase containing dodecylammonium chloride (DAC), and a UV detector. The DA+-coated monolithic ODS columns show greater retention for iodate and weaker retention for iodide, compared to dilauryldimethylammonium (DDA+)-coated monolithic ODS columns. Six anions were determined within 12 min with baseline separation without interferences by matrix ions, such as Cl- and SO42-, in seawater, compared to a DA+-coated particulate ODS column (it took 25 min per sample) obtained previously. The detection limits (DLs) were comparable to the DA+-coated particulate ODS column: IO3- (7.7 μg L-1), BrO3- (20), Br- (88), NO2- (0.9), NO3- (1.9), and I- (0.9) with a 100-μL sample injection. A larger sample volume injection lowered the DLs of IO3-, BrO3-, and I-. The good performance was maintained for ca. 2 years examined. The robust IC system developed in the present work was successfully applied to real seawater samples without sample dilution in the recovery rates of 93 - 104% for all ions.