Wasn't that pretty much just The Abyss/Deepstar Six (the movies), only with Deep Ones?
It was a little like the Abyss (in terms of the undersea environs, subs, etc.), but I haven't seen Deepstar Six so I can't comment there, sorry.
I was quite impressed by the "In medias res" adventure that appeared in the Unspeakable Oath magazine #10.
I got to playtest that before it was published---also an excellent advneture, but it does offer some very difficult roles to play (a mute, in the case of the PC I had), and all of the PCs are somewhat more intense perhaps for a first-time group of CoC players (I guess it depends on your players, but if they're not used to playing escaped mental conviction sociopaths it may not be an ideal intro of the game....).
Thanks for all the suggestions. Grace Under Pressure sounds perect, unfortunately there does not seem to be a (legal) source for .pdfs and print copies are selling for $40 on Amazon.
I'll take a look at some of the other options.
If you do track down a copy of "In Medias Res" then this thread @ Y-S.com is worth reading for some updates/clarifications on the stats:
GM question for In Media Res and here's a post I made a few years ago on Grognardia about running GUP and IMR:
I played GUP and "In Medias Res" with John and the Pagan crew when Pagan was still located in Columbia MO, as well as "Deep Sh*t" before Biohazard published Blue Planet: "Deep Sh*t" is very similar conceptually to GUP, although it's got a few added twists (PCs = GEO SuperTroopers instead of normal geeky scientists, and they're heading into an Alien-like bug hunt where they're the hunted...). I've always thought that GUP and "Deep Sh*t" both worked best when special effects were used: Pagan rented adjoining hotel rooms, and when the party split, they were in different rooms; we used glow sticks for light; they had ocean sounds going in the background, sonar pings, and such available via a Mac audio system; separated parties communicated by walkie talkie (with all of the vagaries of reception thrown into the mix!); etc. I can't imagine playing either game at the dining room table, after the immersive experience that they provided.