Use the tag natural=coastline and draw your island counterclockwise, if you want create it within your maritime borders. To create and island in the river aquatory use tag waterway=riverbank and multipolygon relation.
I don’t know if you mean to build islands inland or offshore. To build islands in an inland lake or river, you need to make a polygon within another polygon - a multipolygon. Like these here.
OpenGeofiction
Discussion
Comment from thermo_nuclear on 16 December 2016 at 12:13
Use the tag natural=coastline and draw your island counterclockwise, if you want create it within your maritime borders. To create and island in the river aquatory use tag waterway=riverbank and multipolygon relation.
Comment from thilo on 16 December 2016 at 18:39
See also: http://wiki.opengeofiction.net/wiki/index.php/OGF:Rules_of_the_sea#Rules_for_island_building
Comment from Ūdilugbulgidħū on 16 December 2016 at 18:53
I don’t know if you mean to build islands inland or offshore. To build islands in an inland lake or river, you need to make a polygon within another polygon - a multipolygon. Like these here.