I had exactly the same issue on my D1800 last year, and eventually we drilled a bigger hole (like 10 centimeter) on the inner skin of the hull to be able to remove the water. I think in my case the water was most probably leaking in from the cockpit, then trickled down under the engine – collecting a lot of dirt and oil on its way and then cumulate between the two skins at the lowest point of the hull. After the water was removed and the leakage was eliminated the bilge luckily remained dry.
But I’m still wondering if there was a better way to remove that water?