This is selection bias. Just because you don't hear it doesn't mean that it doesn't happen.

We only hear about diseases with land animals because that's the diseases that we do researches on. We do researches on them because they are the animals that are important to us, one way or another.

Diseases on animals that are similar to us affects us directly because of cross species infection. Also, we don't tend to hear or care about animal diseases unless it's economically important. Land animals are farmed at a much greater scale, so we care about them more.

Diseases with sea animals happen aplenty, we just don't know or care enough about them to do the research needed to actually notice them.

Simple as that.