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The natural radioisotope radon (222Rn) has been widely used as a tracer for quantifying submarine groundwater discharge (SGD). Applicable equipment for continuous radon-in-water detection using a radon-in-air monitor includes the "RAD AQUA" and the "Water Probe" (both Durridge Inc., USA). Although both devices have been applied to measure 222Rn concentrations in coastal water over time periods from hours to days, the monitoring results for the same water body may differ due to the different physical driving forces for 222Rn degassing from the water, which will result in an inaccurate SGD estimate. Here the two approaches were simultaneously applied for monitoring 222Rn concentrations at the same site in the same surface-water body (Daya Bay, China). The SGD fluxes resulting from both approaches were evaluated and compared, based on the obtained time series and a 222Rn mass balance model. The results demonstrate that the average 222Rn concentration measured by the Water Probe is only ~30% of that detected by means of the RAD AQUA. The mean SGD flux estimated based on 222Rn time series measured with the Water Probe is 69% of that detected by means of the RAD AQUA. The findings of this study suggest that the RAD AQUA can capture more accurately the signals of variation of radon concentration, and is a better approach for continuously monitoring radon and quantifying SGD in coastal zones. Thus, previous studies based on 222Rn data from the Water Probe for estimating SGD and the associated nutrient flux may need re-evaluation. [ABSTRACT FROM AUTHOR]