Fish somewhere else, it means go away and do not fish here. Will you stop trying to fish in my water: means: this is private property or area do not fish here you do not have permission

It also might be a metaphor, depending on the context - it might just mean stop what you are doing because you should not be doing it and it is wrong. If a doctor with many regular patients finds out there is a new doctor in town who is taking all his patients away, he might say to that new doctor "Stop fishing in my water." In other words, stop taking my patients.