If your fishing pools your bobber needs to be in the pools highlighted area. Thoug sometimes Im just outside and it seems to be close enough. There is no aim. I hotkeyed my fishing to 1, but did it on a lower tier of your hot bar. If you hold shift and use the mouse wheel to scroll down you will switch between different pages of your hot bar. Use a lower one for fishing. Then just stand near the pool and hit your hot key over and over till it lands in the pool.
Tips:
Stand with the pool in front of you.
Watch out for mobs, you can fish while standing in water as long as your not swimming. If you get an error saying you didnt cast in fishable water but its water thats fishable you prolly just need to adjust your guy a bit and cast again.
Learn the average distance a fisherman cast at. While sometimes you will cast longer or shorter there is a medium range that is more common. Learn it and stand this distance from the pool. Continue to hit your hot key till you cast in the desired area. Honesty I very rarely have to cast more than 2-3 times while following my own tips.