Hi Apollinaris,
Osmarender has problems with mp on the tile boundary of a rendered area. you can check the tile size at zoom level 12 to see if the lake crosses it. It happens even for very small mp but it doesn't happen all time.
Thank you for your insight. At http://www.openstreetmap.org/?lat=62.896&lon=27.6889&zoom=12&layers=0B00FTF the coastlines are crossing such tile boundaries. Saimaa is a huge lake, even after me splitting the waters :-) I reported this issue at http://wiki.openstreetmap.org/wiki/Osmarender/Development because I knew no better place. I would not yet split the multipolygon further if it is clearly a tool issue. Mapnik and OpenCycleMap look fine, but it is currently unclear if they are is drawing the water from old coastline-based shape files or from the multipolygon definitions. Best regards, Marko