If anything, it's quite the opposite: water, especially salt water, is a paradise for viruses. It's estimated that something like a quarter of all cells that die on Earth are bacteria in the ocean killed by phages. I think maybe what you're sensing is actually more about the rate of human contact: maybe we tend to care less about fish diseases because we'd only really get it from eating uncooked fish (or like, drinking water from a fish tank, which I don't advise).