I posted this in the "visible fish" thread but
@JP_Finn suggested I make a new thread.
I had an idea that may be more simple to implement than new graphics, it would work in a similar way to predicting the weather. You could "activate" your fishing skill next to a body of water and it would give you an idea of how good the fishing is in the nearby area. Not sure how large of an area exactly, maybe as small as 20x20 zoomed in squares to as large as a single zoomed out square. For example, a low skill fisherman might get "you see a few fish swimming around, this may be a good fishing spot". A high skill fisherman may get "this area looks good for casting a line to catch larger fish, but not good for setting a net". Skill would dictate how much information you can get as well as the accuracy. Obviously a low skill would be less accurate and give somewhat vague information. Perhaps under 20 or so it gives no real information and returns something like, "this spot looks as good as any to catch a fish". It may also help with judging fish behavior, such as spawning salmon not being interested in bait and must be netted or speared.
Using the skill would work in a similar way to weather or herblore and would give slight skill increases as you use it to find fishing spots. Since it uses a mechanic that already exists for weather and herblore, adding it may be somewhat easier than other options. It would also make sense logically within the game, since an expert fisher would never just cast a line or set a net, they would first find a good area. It may not assure success, but it would be better than a guess.