Lake Batur in Bangli Regency, Bali Province is a priority area for ecological quality restoration in Indonesia due to its vulnerability to environmental changes caused by diverse and intensive utilization. Agricultural activities, fisheries, and tourism affect the lake’s water quality, leading to the uncontrolled growth of water weeds such as water hyacinth (Pontederia crassipes). To determine the relationship between water quality, community activities, and water weed presence, water samples were collected from 9 locations consisting of residential, tourist, aquaculture, and agricultural areas. In situ analysis was then conducted to measure turbidity, pH, temperature, dissolved oxygen (DO), total dissolved solids (TDS), total nitrogen, total phosphate, and ammonia levels. Results showed that pH, temperature, DO, total nitrogen, and ammonia levels remain within Indonesia’s Grade I water quality standards, which means that water from the lake is still suitable for drinking and domestic use. However, TDS levels ranged from 712 to 1,950 mg/L in the first sampling and 1,340 to 1,950 mg/L in the second sampling. Additionally, total phosphate levels reached 0.37 mg/L, exceeding Grade IV water quality standards, indicating that the water is only suitable for crop irrigation and similar uses. Although Lake Batur’s water quality index was recorded at 63.64 (moderate quality), it remains highly polluted by household waste, pesticides, chemical fertilizers from conventional agriculture, and waste products from fisheries and livestock farming. Since high phosphate levels trigger the growth and spread of water weeds such as water hyacinth, it is crucial to implement measures and policies to reduce pollution in the areas surrounding Lake Batur, which will help prevent disruptions to the lake’s aquatic ecosystem.