My guess is that it would be one of the least likely modes of transmission of almost any vertebrate transmission scenario (I'm not sure on amphibians, they even may be less possible), given the strong innate immulogical protection that fish have: a more diverse array of immunoglobins, but a less developed adaptive immune response compared to mammals. As you say, the slime layer is that innate protection that provides strong, but non-specific or adapative protection to viruses and bacteria, both of which are numerous in variety and numbers in an aquatic environment, yet fish fend them off succesfully.
The ACE2 receptors go way back in all vertebrates I believe, but there is a fair evolutionary divergence in them.
I don't know that we want to talk about the more likely scenarios (or the origin) for potential animal to man transfer (depressing).
The only good new is that COVID-19 has limited diversity so far, unlike the flu.
The fishing related aspect to this thread is that limiting your handling of fish and preventing removal of slim is so important if you plan on releasing them: once this layer is compromised fish are severely disarmed in terms of immune protection. Secondly, if you want to use a bump board, make sure is is wet, smooth (so it doesn't remove slime) and clean thoroughly after each fish, as essentially you are sharing slime and whatever pathogens are in that slime between fish. Don't allow fish you are going to be released to roll around on carpet, gravel etc. Rubberized nets vs old school woven.
I recommend getting one of those fish grippers and leaving fish in water as much as possible if measuring or pics are important to you. I like measuring fish too, but once you have a feel for the size nothing offensive about a good guess and then try to reserve measuring for PBs
Heavy topic, abou