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
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// XXX: What if the monster was invisible before submerging?
if (!quiet && you.can_see(this))
// Don't worry about invisibility. You should be able to see if
// something has submerged.
if (!quiet && mons_near(this))