What he said. Rockwool will stay much too wet if it sits in water. Your plants will rot. Some roots in water all the time can be good however, especially if you have power outages. Expiriment with on/off flooding times. You goal is to have the roots as dry as possible without stressing the plants.

Clay pellets in my opinion are much better than rockwool. Not so many moisture-related problems, and completely reusable.