what you may be able to see will be dependent very much on what type of habitat you are fishing (marine, freshwater, open ocean, over sand, reef etc) as what you may see is very much influenced by how deep the fish may be, how clear the water is and whether the fish contrast with their surroundings. Many have camouflage and can even change colour when they swim from one area to another (e.g. from sand to reef) . As mentioned earlier, with the exception of a few large species of tunas and sharks, fish are cold blooded and wouldn't show up on infra red. The polariser is a good idea for sure as the angle of your aircraft and sun glare have a big impact on what you may see.
There were a lot of fish moving along the beaches near where I live over the summer. I didn't need a drone to see them but did take lots of video looking for sharks in amongst them (didn;t see any). Whilst there are good photos around of sharks etc they are very difficult to search for using a drone. I spent quite a few years in a light aircraft at 400feet looking for sharks over hundreds of kilometres and they are uncommon and challenging to spot so doing it from a drone I think is even harder. Its more a matter of either luck or seeing one on the surface and then putting the drone over it. In my experience surface fish are easier to see with the naked eye and fiddling about putting a drone up and over them takes time away from casting a line.
Here's a few sequences that may be of interest.
This was a random find, fish just hanging out on the surface -
- see minute 2 or so
See Minute 2 or so -
See Minute 2, 3 (birds over fish) and 3:30 (not sure how I did this) -