Jet A dissolves water easily, so with jet fuel you do have to let stuff settle for an hour or so (or so they've told us).
The sump is ideally the lowest point in the system you are trying to sump. The problem is, cessnas have a rib underthe middle of the tank (or maybe a few anyway) and over time the tank will sag around it. Water can then be left in the new depression and not sump out.
We had a 152 with the flight team with this problem (the others didn't). Anyway, this particular cessna ended up having a fuel cap go bad. It wouldn't let fuel out but would let water in when it rained. You'd sump the plane and not get any water, but around climb out after a touch and go the engine would try to die. The turns around the pattern would be enough to get the water out of the trough that had formed in that plane. I think they got over a half gallon of water out of it that wasn't sumping out once they finally figured out what was going on. That plane we'd rock the wings really good before each flight... the other ones we tended to do that with too, but they didn't have that problem so it didn't make a difference on how much water you got out.
I'd like to note that I've noticed a LOT of people miss belly sumps on cessna 152s and 172s. Some of them have that sump plugged instead of actually having a sump there, but it seems most people don't know that there *can* be one there.
As far as Av gas and water, I don't think water even tries to dissolve in avgas... IE, it'll come out rapidly if it was suspended at all. A good way to test this would be to get a bottle and put some water and some gasoline in it. Shake it up and see where the water goes.
As far as jet fuel and sumping... we don't sump our planes... self defeating if we are going to fly the plane immediately after fueling 99% of the time. Guess it's a difference thing with turbine and piston ops. I'm sure the DA-42 twinstar will have water seperators on it (standard equipment on automotive diesels).
