I've broken even or made money on every waterfiend task I've ever done. The noted water orbs, water battlestaves, rune meds, and noted snape grass add up.
Not a problem with which spot you're using then....are you making sure to lure the other two that can reach you? If you do you should never be ganged up on.
They have a limited amount of food and can only walk through doors. To OT: I remember Jagex stating that the reason they didn't want revenants to have good loot is because they didn't want people hunting them, so I doubt they would change that.
I would stick to bursting. Constantly using wolper scroll to barrage would get annoying, plus it occasionally attacks one of the lobsters causing it to break off from the lure.
Unless you're doing FFA, you'll be crashed almost instantly, if you can even find a world in the first place. Even then your trips would be rather short and you probably wouldn't be making much.