I don't remember much about it because I was so young at the time. I would say it came out somewhere between 80s to early 90s judging by the art style.
Things that I clearly remember are:
The main character was a teen boy with brown hair. He might have had a younger little sister there.
Fighting a huge fish (green or yellow) looking monster that could fly. Imagine a big-ass catfish looking thing.
Actual fight was some large cliffside surrounded by sea
Can't remember if there were mechas, planes or if he was using a fishing rod.
It's not much to go on but hopefully someone recognizes what I'm referring to.