It is a multipolygon first and foremost because it is land cover and the best way to do land cover (and land use) is multipolygon, though second of all it is a multipolygon is because parts of it span large areas so editing one of the ways that make up its edge does not make the changeset geographic area unnecessarily large, and last but not least the multipolygon represents the whole of the area the canal takes up and since it is one canal it is one area and thus the best way to represent it, as one single object in the database. 

Again adding non multipolygons attached to a multipolygon makes it significantly harder and time consuming to fix and update so I would greatly appreciate it if you would do them as multipolygon from the get go. Again this is land use and thus should be done as multipolygon. 