So I figured I'd see how effective fishing actually is. After expending the 50 worms the beginner rod comes with (which is pure profit) I did some wormless fishing for a while. Waiting ~8 minutes between casts sounds bad if you're just sitting there watching your avatar, but I did it while drawing, so my time was occupied. I would draw a little, then switch back to my Firestorm window and click to recast. It was slow but still pure profit. You get about as much L$ and fishing experience from wormless fishing as you do from small worms, just slower. You can make more money in less time with Gold Hunt, but that requires me to pay attention to my SL window. Sometimes I'm doing something else, so fishing is good to have around.
So to see if worms really mattered, I went and bought 100 small worms for L$22. To make that money back, 100 casts would have to average out to L$0.22 per fish. They don't! Every so often I would get 0.4s, or even once an 0.9, but most casts got me 0.1 or so. But!!! Every so often you win extra worms from a fish lottery. So my 100 casts was probably more like 130 (I wasn't actually counting), and I ended up with L$26 when my worms were finally depleted.
4 lindens isn't much, but it was fast. Also, I have less than 500 fishing experience, so potentially I could get more from the same investment if I keep going. Also also, I'm using a beginner rod. Rank bonus on a deluxe rod might have made a significant impact.
But I can fish with the beginner rod and no worms for free, and get the same experience gain. So that's what I'd recommend is to do some wormless fishing when you idle. If you're doing something in another window, like watching youtube or whatever, just set yourself up so you can cast every 10 minutes or so. Buy worms once and a while when you intend to chat in SL while you fish, so that you're not staring at the same boring cast for a long time. And eventually you profit!
I have no experience with fancy rods or shubbies or worm farms or whatever, but you can still make money doing next to nothing at all, if you want.