I watched this movie many years ago, but it always seems to elude me as to what the title is.

Here is what I remember from the movie:

-it took place on a oil rig in the middle of the ocean
-it had a shape shifting alien that took on the desires of the person in the vicinity
-the alien came from a rock at the bottom of the ocean
-it starts with the rock breaking open and killing all of the scientists in the lab

Sounds like V but I think they came from space

There are a few that could match what you're looking for:

DeepStar Six (1989) - IMDb

Leviathan (1989) - IMDb

Or this newer one:

The Rig (2010) - IMDb

Hmm I'm sorry to say that none of those match what I'm looking for. Though they are very close.

If it helps I also remembered that a marine/mercenary had a huge huge obsession with an adult star.

The only other one I can think of is

The Abyss (1989) - IMDb

Ok thanks for your help. I'll check out the movies themselves rather than the trailers just to be sure. Once again thanks for your time.

Possibly Sphere -
http://en.wikipedia.org/wiki/Sphere_(film)
Sphere (1998) - Overview - TCM.com

This one has a shapeshifter:
Proteus (1995)
IMDb synopsis:
Heroin smugglers find shelter on an abandoned oil rig after their ship explodes. Soon, they find that the oil rig was just cover for a biological experiment. One of the results is Charlie, a shapeshifting creature with the ability to absorb the memories of its victims.

Proteus (1995) - IMDb


This one isn't a shapeshifter, but I thought I'd suggest it just to rule it out:
The Intruder Within (1981 TV movie)
IMDb synopsis:
Drillers on an oil rig near Antarctica discover that they accidentally brought up several prehistoric eggs. One egg hatches and becomes an unstoppable creature. Two workers (Chad Everett and Jennifer Warren) risk their lives to stop it.

The Intruder Within (TV Movie 1981) - IMDb