The first thing that springs to mind is water, but the water has to be the right type (freshwater, estuarine or saltwater), temperature, quantity (depth and flow) and quality. Fish habitat is more than just the water.
There are other things that make up the habitat fish need to survive and thrive. For example:
At each stage of the life cycle each type of fish requires a particular habitat to survive and thrive. Only by maintaining this range of habitats can we make sure we have healthy fish populations into the future. That's the basics, but fish also need different habitats for spawning and growing out as well as to be able to migrate.