<p>Due to the special characteristics of low octanol-water partition coefficient and high-water solubility, melamine (MEL) and its derivatives released from daily products will be preferentially allocated to aquatic environments. However, no study has reported the occurrence and distribution of MEL and its derivatives in the diverse aquatic environments in China. This study collected 8 categories of 121 water samples in different regions of China, including bottled water, tap water, swimming pool water, river and lake water, precipitation, well water, filtered water, and network pipe water. The ultra-performance liquid chromatography–tandem mass spectrometry was used to measure the concentrations of MEL and its three derivatives, ammeline, ammelide, and cyanuric acid (CYA). The hazard quotient (HQ) and hazard index (HI) were utilized to evaluate the health risks posed by MEL and CYA in water samples for the general population or aquatic organisms. In addition to bottled water, the detectable rates of MEL and its derivatives in other water samples exceeded 94.87%. The highest average concentration of MEL was observed in well water (181.371 ng/mL), and the highest levels of CYA was observed in swimming pool water (14,477.650 ng/mL). In composition of water samples, the primary chemical constituent in well water and precipitation was MEL, whereas CYA emerged as the predominant chemical constituent in other types of water samples. The external exposure assessment indicated that except for well water and swimming pool water, the calculated HQ and HI values based on the concentrations of MEL and CYA in other water samples were significantly below 1. This study demonstrated the widespread presence of MEL and its derivatives in various aquatic environments in China. The general population may elevate their exposure risk of MEL and CYA through the daily consumption of well water or unintentional swallow of pool water during swimming.</p>