Analysis of seawater quality variation and its influencing factors can be used to produce pollution management policies and help to evaluate the effects of nutrient reduction efforts. In this study, the generalized additive models for water quality trend analysis were established based on the dissolved inorganic nitrogen (DIN) concentration and precipitation data of Tianjin coastal waters for the 2007–2018 period. The variation of DIN in 12 stations located in Tianjin coastal waters and the response to precipitation were evaluated. The contribution of precipitation was distinguished from other possible driving factors. Results showed that the water quality in the coastal waters of Tianjin was gradually improving in 2013–2018, compared with 2007–2012. At the 95% confidence level, the DIN concentration in 7 of the 12 stations was declining, one was on the rise, and the other four stations showed no significant change, indicating that the pollutant emission reduction measures have achieved initial results in improving the sea water quality. The change of DIN concentration in all stations was less affected by precipitation, mainly controlled by other factors. The farther offshore, the less the impact of precipitation on DIN concentration in the sea water. The change in DIN over the period of record ranged from a decrease of 39.95% to an increase of 29.79%. The coastal areas were divided into five regions based on the analysis results. It is suggested to strengthen water quality management in the north and south districts of coastal areas, enhance the prevention and control of water pollution in the associated watersheds and reduce nutrients into the sea, in order to reverse the deterioration trend of water quality in the two regions.