My grey water tank has been integral to the steel hull since 1963. No problems yet. I did clean, repaint, and replaced the suction line in it last year. The suction line had pitted and a couple of holes in it.
dan
An integral grey water holding tank is a great idea if you have the option of installing one.
Not only do you not mess up marinas and confined bays, but it offers another layer of protection if that part of the hull is damaged.
It would be worth coating with a good quality protective coating.
The Ulysses grey water tank is about 250 gallons and is pumped overboard through one of two pumps that are both fire pump and major bilge pumps depending on what valves I open....