ROSE: Her husband had struck gold someplace out west, and she was what mother called "new money."
This is Rose describing Molly Brown. Molly seems like good people to us, but people like Rose's mother are always quick to distinguish themselves from "new money." Plus, Ruth thinks Molly is vulgar to boot…
TOMMY: Ah, forget it, boyo. You'd as like have angels fly out of your arse as get next to the likes of her.
When Tommy sees Jack checking out Rose, he tries to quash any hopes he might have of getting with the young beauty. Since Rose is clearly of a different class, Tommy doesn't think Jack has a chance.
CAL: What made you think you could put your hands on my fiancée?! Look at me, you filth! What did you think you were doing?
When the ship's crew mistakenly thinks Jack had been attacking Rose (when he was actually trying to help her), it's understandable that Cal is upset… but he definitely shows his snobby stripes in the language he uses to communicate his anger.
CAL: Well, it's for royalty. We are royalty, Rose.
Cal gives Rose a huge-tastic diamond necklace called the Heart of the Ocean. He senses that she hasn't been feeling all that chipper, and so he decides to give her something really expensive and elite…since, you know, that's the kind of thing that would cheer him up.
UNNAMED COUNTESS: Look, here comes that vulgar Brown woman.
RUTH: Quickly, get up before she sits with us.
Remember how we said Ruth didn't like Molly? Well, this is the kind of thing we were talking about. Ruth is trying to escape from the lunch table before Molly can join her and her friends. Ruth does not consider Molly to be on her level in the least, and she barely hides her disdain.
MOLLY: Well, you're about to go into the snake pit. What are you planning to wear?
It seems that Molly doesn't think that highly of Ruth and her friends, either, since they are the members of the "snake pit" she mentions here when talking to Jack. You see, Jack is supposed to come to dinner with Rose, Cal, etc., as a thank you for saving Rose's life…and as Molly points out, he'll probably need something posh to wear.
CAL: Dawson! That's amazing! You could almost pass for a gentlemen!
Cal manages to insult Jack while complimenting him, of course. Sure, he seems impressed at how well Jack cleans up…but still says that Jack can only "almost" pass for a gentleman.
ROSE: They assumed he was one of them—heir to a railroad fortune, perhaps. New money, obviously, but still a member of the club.
In voiceover, Rose describes how the posh types reacted to Jack's appearance among them. Apparently, he passed quite nicely for a gentleman (Cal's little joke aside).
RUTH: Tell us of the accommodations in steerage, Mr. Dawson. I hear they're quite good on this ship.
In an effort to remind Jack of his real "place," no doubt, Ruth's first question is about how he enjoys steerage. "Classy."