Right now, the best way I found is this:
1. cut a point from both river side.
2. create a line between both river side.
3. remove the relation for each river side
4. create a closed-polygon with all lines.
5. select the river polygon and each island
6. remove relation for island
7. In tools menu, create a multipolygon
8. Select the river polygon and add "natural=water".

changeset/1965592 deleted (see changeset #1971696)

changeset/1965585 deleted (#1971294 #1971405 #1971536)

changeset/1965580 deleted (see changeset #1971087)

changeset/1965578 deleted (see changeset #1970800)

changeset/1965505 deleted (see changeset #1970277)
changeset/1965509 deleted (see changeset #1970469)

It was finally changeset #1969560 for cancelling #1965429.

changeset/1965469 deleted (see changeset/1970056)
changeset/1965491 deleted (see changeset/1970164)

#1965595 is ok if I import HD_WATERBODY_2 1, 2, 3, 4 in the same order
I'm currently deleting changeset #1965429 (see changeset/1969974)

You need to install or compile webkit-image (a web downloader).
osm.wiki/JOSM/Plugins/WMSPlugin#webkit-image_.28QT.29

I find that Qt is easier to compile when you have all libraries.

is svn1529-1 the same as http://packages.qa.debian.org/j/josm.html ?

Oh, i've found that when I import some data from openstreetmap server (a little zone), I can commit my change without any NullPointerException.

I had this problem with JOSM 1727, I could easily fix it if I know how to change node long/lat manually and send the changes...