Yeah, fishing is easy if you can see the fish underwater. It's really obvious when the fish is about to bite so you just need to time your RMB click-hold with that.

Demo:
https://www.youtube.com/watch?v=cw8Fx51w9Sw
Another way is passive fishing or trolling. When you're getting headwind during your boat trips, just cast the line and sail at speed 1. Might as well catch some fish if you can only move that fast. If you're sailing along the shoreline, you can easily catch pike and tuna this way.