The temperature of ice stays very close to 0°C. As a storage medium for fish, it is perfect at keeping the fish in peak condition without freezing it.
Gel packs need to be frozen to a very low temperature in order to store sufficient energy to do their job. The most modern trawlers still use ice on the fish, even though many of them have dedicated refrigerated fish rooms. The extra refrigeration helps to hold the temperature down, but the ice does the important job of cooling the fish straight from the sea.