The newer plastic filter housing has an unusual arrangement there is a threaded plastic plug that fits down the side of the filter at an angle of about 85 degrees, be very careful removing it it breaks easily.
As with all questions on the forum, to get an accurate answer the Model & year of manufacture of the Ducato & which engine is fitted needs to be stated. If it is an 02-06 244 Ducato then the water sensor on the bottom of the filter has to be disconnected & unscrewed to drain the water. As corkman has stated I think the new X250 uses the plastic filter, but I would immediately replace the filter itself if it has water in it regardless of which model Ducato it is.
Cheers