Refers to water from the sea, often in contrast to freshwater. Can be used in contexts like cooking, bathing, or describing the ocean.
Gargling with seawater is said to be good for the throat.
The seawater around here is very clean.
Compound of 潮 (しお, tide, seawater) and 水 (みず, water). The reading しおみず is a straightforward combination of the kun'yomi readings.