I think there's quite a few things wrong with the conception and execution of this... as you've been asking for feedback on how this kind of game might work out, I assume you want some feedback on this one, so I'm gonna go all BlueScope-style ranting on it...
First of all, as rosareven already mentioned, the hitboxes don't work out at all. The 'line' (heh, I see what you did there!) itself is maybe a bit too thick as well, and because of the blockyness to it, you expect to get an underlaying grid, which makes you press the direction earlier than you would've wanted to... probably even without knowing it.
Because of your hitbox handling, I'm going to go ahead and say that you're not checking for spawning locations, meaning a fish could possibly spawn under the line without even being seen.
Then, the concept of just starting over won't work out too good, I think. Snake used a highscore to make people try to get the snake as long as they possibly could, however with your version, there's no drive to do that whatsoever. You have the easiest time just going for the first fish, running into the next wall, start over... and it even takes about the same time to do that.
What you maybe could do instead is some kind of multiplier that will - for unexplained reasons - give you bonus fish if you manage to keep the line in the water for a longer time. Since you don't have a "fixed length" snake either, but an unlimited length one, it raises the difficulty by a lot (what's the maximum expected caught fish, maybe 10 or 15?), so that might be a catchy idea.
Another one would be to make the player stop to actually get the fish... or even better, create a zone where they have to reel the line in to capture the fish; otherwise, they'll lose them. I'm wondering whether or not that's gonna make it too complicated, but I think it's worth getting a tad more complexity in it compared to the stale "just try again, losing doesn't hurt you at all" mentality it has right now...
On a random side note, I really like the announcement-with-face thing you got there... I'm unfortunately not familiar with the rest of your game, so I don't know if you have those for everything... but it really makes the instructions way more interesting to read than just a bunch of lines. Don't know why I notice this now for the first time :D
If you have a slightly positive memory of my Power Shift contest game,
you might be interested in this development screenshot
More info about that soon!