Hi aighes,
I replaced the natural=water from the outer ways to the multipolygon-relation. Same error I had with Vuohijärvi. Maybe this works better.
I guess you are suggesting that if some of the three role=outer ways of the Vanajavesi coastline had natural=water set, mkgmap could close these lines, assuming them to be polygons? I just checked (ctrl-L in JOSM, and loaded http://api.openstreetmap.org/api/0.6/relation/302922/full). None of the relation members have anything set, except place=island and name for two islands: http://www.openstreetmap.org/?lat=61.1801363&lon=24.0677991&zoom=16 (I guess place=islet would be more appropriate than place=island, given that large populated islands in Helsinki are tagged as place=islet.) Marko