My immediate reaction is NO but, as almost always is the case, there are some exceptions. Municipal tap water (ie not from a well) contains some form of chlorine (the worst being chloramine (a combination of chlorine and ammonia - considered safe at human ph levels but not so much so at the higher ocean approximating levels. You will need to check on which is used by your specific county). Additionally, some water will contain unacceptable levels of copper (trace amounts can be deadly to octopuses but not to scaled fish). Most octopus keepers either buy or make RO/DI filtered water.
I am not sure. I suspect that maintaining a high (8.3) pH may be more difficult (I seem to have this issue even with running my RO water through a DI filter) and it may be harder to fully dissolve everything in the saltwater mix but I don't know of any direct issues.