Our Super Maramu has a drain under the galley sink that goes directly into the sea. There is an option to divert the grey water from the sink into the sump. There is some gurgling on starboard tack but that is the only downside as far as I can see. I wouldn't have it arranged any other way. The sump is so much easier to clean and needs cleaning a lot less frequently.
The only problem is that any animal fat that you allow down the sink will meet seawater and if it is cold enough it will congeal and potentially block the sink. This happened to us once. Since then we wipe as much fat off the plates as possible before washing up.
I didn't install the drain but it is a very simple set up. The through hull is an inch and quarter.
Mark & Nicky Barter