* Describing water: "The coastal lagoon was filled with brackish water, a mix of fresh and saltwater that made it difficult for most fish to survive."
* Describing a taste: "The oysters had a slightly brackish taste, a subtle hint of the salty ocean water they had filtered."
* Figuratively: "The atmosphere in the room was thick with a brackish tension, a mixture of excitement and apprehension."
Let me know if you'd like more examples!