Great question – and I think the answer is that we just don’t know! It’s only relatively recently that’d we’ve started to look into aquatic diseases, and I think it’s a case of the more we look, the more we’ll find. If I had to make a guess, I’d say the land animals probably win – just because we’ve been studying them longer.
As Bethany says that’s a hard question to answer as we actually know more about the moon than we do about the oceans on our planet.
There are thought to be more creatures in the ocean than on land so unlike Bethany I am going to say that diseases affect ocean creatures more just because there are more of them to infect.