The following is generic and there may be specific local regulations that tighten it up, but I don't know of any, at least on the east coast, SoCal and the PNW where I have boated.
There are three catagories of marine discharge zones:
1. Outside the three mile offshore limit anything goes essentially- sewage, grey water, etc can all be dumped.
2. General inshore discharge- Sewage can only be discharged if it has been treated by a Raritan lectrosan or purosan or equivalent to sterilize it first. Grey water can be discharged.
3. Zero discharge zones- These are set up by the EPA and maybe local authorities to protect sensitive areas. For example I believe that the entire Chesapeake is zero discharge. In these areas no sewage can be discharged but all grey water can be discharged.
So again AFAIK, grey water can be dumped in all navigable sea water areas in the US. I suspect that the ambiguity you are seeing is to scare you off.
Also EPA regs require that any overboard sewage connection be secured. Some have interpreted this to mean a lock and key on the thru hull, but the practical application is to wire tie your discharge valve handle shut or remove the handle completely. The only place where I have seen a positive requirement to block the flow path overboard, ie remove a piece of hose is in Lake Champlain. They are anal ;-) for good reason and it isn't sea water in any case.
David