I have not implemented a macro like this but I can confirm that IF you have a bobber active in fishable water, recasting will NOT have any effect besides the error message IF the recast would not have given you a clickable bobber. That is, if the water is too shallow or whatever. This is tested, observed, and true for hotbar-clicked and keybound casts (which I have been using in Dalaran).
Casting while you have an active bobber WILL move the bobber and restart the timer if the recast also lands in fishable water.
I don't see much use for this in the Extravaganza, where you are trying to hit the Tastyfish school and are casting into generally fishable waters. You loose time in the Extravaganza if you recast a "good" bobber. However it would work nicely in places like the Dalaran fountain and fishing Iornforge lava, especially from a "poor" angle (one in which you get lots of miscast errors). There's at least one Tastyfish pool I recall which is in a buggy spot, lots of shallow water errors making the school hard to hit -- I may rev this macro up next time I go to the Extravaganza.
So.. it could just as easily look like this:
1st cast lands in shallow water and gives error message.
2nd cast lands in fishable water, bobber appears.
3rd cast lands in shallow water and gives error message, however, because it didn't land in fishable water, the bobber from 2nd cast remains active
4th cast lands in fishable water, bobber moves (dissapears from second-cast spot and reappears with fresh timer).
...Or four shallow water errors if you're REALLY badly situated or unlucky; or four separate casts, each with a "new" bobber and a new channeling timer, if you don't hit an unfishable spot. Thus, a timesaver (and nerve-preserver) in hard-to-fish spots.