Fish do have quite a lot of worms and other parasites, fungal and viral problems, in fact. Some studies suggest marine animals might be more susceptible to diseases due to environmental chances and population densities. Some studies suggest marine animals meet more microbes, while dryness, UV and some other factors reduce half life of terrestrial microbes. However, it is a difficult comparison. Moreover, how do you want to define "disease"? The definition Google gives is "a disorder of structure or function in a human, animal, or plant, especially one that has a known cause and a distinctive group of symptoms, signs, or anatomical changes." I assume pollutants, temperature and so on cause more diseases in the marine environment, at least I hear more of them.