Cockpit drains below the waterline is what sinks boats. Especially in the North country.
Just like David said, the only outlet below the water
should be the head
and that is for sanitary reasons. And it should have a seacock which is shut off when the boat is not occupied, as well as the other under water fittings.
I do have a sink drain straight down only because the sink is only 1' above the water line so I don't have a choice. But the valve gets exercise regularly.
One problem with under water fittings is that sea life likes to make their homes in there. I keep my boat on the hard
most of the time but even the last time out for a 3 day weekend I had sea creatures trying to make a home in my head inlet. They got caught up in a stainer.