Just a stupid thought that entered my head for some reason.
there are many fungal diseases of fish, so I suppose the answer is yes, at least in fresh water
http://edis.ifas.ufl.edu/VM033
Yes, there are quite a few species of marine fungi that live on plants, algae, dead wood, etc, and some fungal diseases of marine fish (and I would guess other organisms). Although marine fungi seem to be much less common and less diverse than in terrestrial and freshwater environments, I don’t think they have been very well studied. They could be more common and diverse than are supposed.