I think I understand what the OP wants.
No, there is NO (legal) way to link the mouse click to a keyboard key.
You HAVE to click the mouse button, Blizzard won't let you link it to the keyboard, period.
Here's a tip on how I fish, rather fast, without using macros or add ons.
I usually do it when wating for friends, but that doesn't mean I enjoy wasting 1 hour for 2 skillups due to slowness.
So, preparation time, I put the fishing icon on the quickbar, at 7 or higher. You'll see why I don't put it from 1 to 6 in a second.
I equip the fishing rod, apply a lure if I need to, then block the SHIFT key in the pressed position with a pen cap or another small pointy item.
Now,
- I press 7 (or whatever over 6) to cast the line (If I pressed 1-6 I would only change the quickbar since the SHIFT is pressed)
- I position the mouse over the bobber
- When I see the bobbing or hear the splashy sound I right click then re-press 7 immediately and reposition the mouse.
This way, I don't waste time either looting or casting, and my hand is not tired from constantly pressing the SHIFT key.
This may be overkill to those of you with less lag, but in my case (700 ms latency) I need to click at the EXACT moment of bobbing not to lose the fish. So I don't have tme to press SHIFT before looting only, I need it on all the time.
Now if I COULD link the right mouse button to the keyboard it would be sooo much simpler, I'd set it to 8, and then happily only touch the mouse to follow the bobber at each cast.
Hope this helps.
Also, hang in there, catching sagefish has added to my fun, and seeing wreckage interrupts anything I'm doing just to fish it.