I’d agree with Tommi and Mark on this, a sea water inlet totally separated from the toilet flushing water inlet would be my choice, and as Mark said, in a location where it is unlikely to pick up anything that may have just been flushed..
I know that some are reluctant to put another hole in the hull, yet many of us are perfectly happy to replace through hull fittings changing from brass, bronze or DZR fittings to glass fibre or plastic through hulls and valves. We would all take great care when replacing a through hull to ensure that its fitting was watertight, so apart from the hole which would be new, for me I really don't see a problem.