ice-water mixture; slush ice
Specifically a mixture of ice and water used to keep fish fresh, often in a cooler or fish box.
The fisherman returned to port while keeping the fish chilled in an ice-water mixture.
Compound of 水 (みず, water) and 氷 (こおり, ice), with rendaku voicing of the second element. The reading みずごおり is a straightforward combination.