Ensuring continuous access to safe and clean water has become a critical global challenge due to rapid urbanisation, industrial expansion, and environmental degradation. Conventional water quality monitoring approaches rely on manual sampling and laboratory analysis, resulting in delayed responses and limited spatial coverage. Recent advancements in the Internet of Things (IoT) have enabled the development of smart water monitoring systems capable of real-time sensing, remote data transmission, and intelligent analysis. This paper presents a systematic review of IoT-based smart water monitoring systems with emphasis on sensing technologies, communication architectures, data processing strategies, and system scalability. Existing research demonstrates that IoT-enabled solutions significantly enhance monitoring efficiency, reduce operational costs, and support data-driven decision-making. However, challenges related to energy efficiency, data reliability, scalability, and intelligent analytics remain open research issues. By critically analysing existing studies, this review identifies research gaps and highlights future directions for sustainable and intelligent water monitoring infrastructures.