Well, digging into the question, I found this discussion in github: https://github.com/openstreetmap/osm2pgsql/issues/994

I've overpassed the whole database, and land masses are divided in two with respect to this tagging: 102 are tagged as type=land_area and 93 as type=boundary + boundary=land_area

I hadn't checked that the wiki hasn't been discussed enough. It has only two versions.

It would be interesting to bring this discussion to the tagging mailing list.

Rafael.