According to what I've found on Wikipedia "The main diet of deep sea demersal fish (This includes Haddock) is invertebrates of the deep sea benthos and carrion. Smell, touch and lateral line sensitivities seem to be the main sensory devices for locating these" However farm-raised haddock are fed fish feed which among other things includes small fish.
Many animals eat seaweed, including hermit crabs, lobster, sea turtles, and marine iguanas. Herbivorous fish like parrotfish, damselfish, and surgeonfish can eat seaweed. Seaweed also is consumed by humans. Lots of different animals eat water weed. These are mainly animals who live in and around water. E.G. Dragonflies Fish Frogs/toads etc. Hope this helped, there are many more animals that eat water weed but this was just a brief understanding. ~:)~