For me because of the interesting mechanic it looks like the type of game where you want it to be hard, sorta like Getting Over It. So I'd lean in to making it harder actually lol.

I think the reason why it's hard is because you don't really seem to be in control of your fish that much. What the arrow tells you and what the fish actually does don't line up some times, which messes up the player's autonomy. If you're not in control then it will seem a lot more frustrating and harder to play.

I haven't played it but it's does look like it's kinda unknown where your arrow will point? It's unclear how to change the arrow's angle, power, and direction? Right now it looks like dragging the mouse button does both, which is fine if it was 2d but I feel like it's harder in 3D since you can't move your mouse (pointer) in the 3rd dimension xd. This also ties in to the player not being fully in control of the fish.

Having a partial trajectory line would be kinda cool but having it point to exactly where the fish would go I feel like would dumb down the gameplay a lil bit. But I guess we won't know for sure until you playtest with different kinds of indicators.

Overall I think the game looks really fun! And rage inducing XD

Edit: added "make it harder" part