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.