Water with salinity between fresh and seawater, often found in estuaries. Technical term used in biology and environmental science.
This estuary area is brackish water, forming a unique ecosystem.
海水 means seawater, which has higher salinity than 半鹹水.
Compound of 半 (half) + 鹹 (salty) + 水 (water). 鹹 is a relatively rare kanji meaning 'salty' or 'brackish'.