[bvw]

No. Here's a list of some that I found w/ google though ... Unix or Unix-like Operating Systems

[dyed_in_the_wool]

I'd think the Navy would either go with a BSD variant (FreeBSD, OpenBSD, etc.), a BSD-Like Linux (Gentoo, Slack) or with a proprietary system (QNX?, Plan 9?)
I would have recommended SCO except that Caldera is screwing that up now.
Not that I don't think Linux is up to the task in general, or embedded Linux in particular.
In fact, in the old License Agreement for Win 3.1 I remember reading a line about how Windows should never be used for mission critical deployments.
'nuff said.