Make messages about monsters' submerging show up again, using the same

logic as for drowning messages (since you should be able to see a disturbance due to the submerging even if the monster is invisible).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9962 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on June 11, 2009
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