Is the sinking liner in the Crimes of Grindelwald the Titanic? If it is the liner, they got the weather all wrong because it was a starry night with calm waters, not the rough sea and lightning they showed.

Almost certainly not.

Even though Leta's apparent age could set the year as 1912, there is no reason to believe that the ship was the Titanic. As you said, the weather is all wrong, and Four-Funnel Liners, while not common, were not unheard of. So it is likely that the ship, as far as the producers were concerned, was a period appropriate ship, and had no real-world correlation.

Additionally, JK Rowling’s original screenplay states that the sinking of the ship took place in 1901.

INT. SHIP’S CORRIDOR—1901—NIGHT

The door of the opposite cabin is ajar.

So at the very least, there appears to be no intention to connect the event to the sinking of the Titanic in 1912.

Looking at Leta’s age, it sure could be 1901, but certainly could not be 1912.

She was born between 1896-1898, as Newt's birth year according to Pottermore is 1897, and they were in the same year at Hogwarts. We don’t know what time of the year either of them are born and so Leta’s birth year can’t be calculated exactly.

She’s obviously not a teenager at the time of the shipwreck. Albus Dumbledore meanwhile was born in 1881 and is clearly older than both Newt and Leta.

No, it almost certainly isn’t the Titanic.

The theory that the ship Irma, Leta, and Corvus Jr. were on was the Titanic was directly addressed by the producer of Fantastic Beasts: The Crimes of Grindelwald, David Heyman in an interview with the Pottermore News Team, where he said it was never discussed.

The mysterious ship is most likely not the Titanic

In one scene in the film, we see a flashback sequence with Leta Lestrange traveling on a ship with her brother – with tragic results when the ship begins to sink. Many fans have wondered if this is the famous sinking of the Titanic, which happened in 1912. David Heyman, however, disputed this.

‘It was never discussed,’ he told us. Although Callum Turner cheekily added, ‘I bet it could have been.'

Mysterious!
_{^{- 10 things we learned from The Crimes of Grindelwald cast and producer David Heyman (Pottermore)}}

David Heyman said it was never discussed, so the movie therefore couldn’t have been made with the deliberate intent of the ship being the Titanic. Also, the Pottermore News Team concludes that the ship likely wasn’t the Titanic. Since Pottermore is officially affiliated with JKR, it’s unlikely they’d have inaccurate information on that, and furthermore, their reporting on Fantastic Beasts has been accurate so far. The year given in the screenplay for the screenplay was 1901, years before the Titanic sank, so the screenplay entirely contradicts that theory as well.

EXT. LIFEBOAT—1901—NIGHT

CHILD LETA, IRMA, and BABY CREDENCE are in one boat, CREDENCE’S AUNT and BABY CORVUS in another.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

It’s likely that the theory was addressed on Pottermore by David Heyman as a way of officially refuting it, since it makes no sense when also considering what’s already stated in the published screenplay.

I thought it was the Titanic at first, judging by interior etc, but if it had been 1912, then Corvus and the other child would only be 15 in the movies. Pretty sure Credence is meant to be older than that and it's him they're considering, after all.