You differentiate seawater, brackish water, and groundwater primarily by their Total Dissolved Solids (TDS) levels:
Seawater: Very salty, typically 10,000–35,000 ppm TDS
Brackish water: Slightly salty, typically 1,000–10,000 ppm TDS
Freshwater: Fresh to mildly salty depending on location (<1,000 ppm TDS)
