Now that the movie is released we can finally really answer it:
it's a combination of projectors and drones, controlled by Mysterio
movies.stackexchange.com/users/19085/Ivo Beckers
I saw this picture on reddit which was interesting.
Looks like it could be hydro-man since the reference is there.
Credit - https://www.reddit.com/r/MovieDetails/comments/agtvu1/do_movie_trailer_details_count/
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He could be named Hydro-Man
As Mr Sunday Movies noted in his trailer breakdown , there is a shot showing a boat with the name: ASM 212 Which is likely a reference to The Amazing Spider-Man , Volume 1, Issue 212, which featured the "Wet Threat of Hydro-Man!"
It's possible that the villian will be named similarly in the film.
Similarly, the other villain's identity may be The Molten Man , evidenced through a number plate that refers to a Spider-Man issue, released on September 28, 1965.
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No one knows for sure at the moment What we do know is that The Elementals are in it.
Because...
well...
Who else could it be?
Magnum and Hellfire for sure are shown in the trailer.
And it'd be pretty confusing to moviegoers if they introduced both Hydro-Man and The Elementals at the same time.
However, they have similarities and so the talk on the street is that it's going to be Hydro-Man in The Elementals, replacing Hydron.
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It's currently unclear, but IGN has a good breakdown of history of the character vs the elements and what they could do and why:
Oh, and there's also a guy called Hydro-Man who may or may not be in the mix here as well, so we'll talk about him too!
(It seems likely that he has been combined with Hydron for Far From Home, or we just aren't sure which water-based baddie
the film is using at this point.) ...
Yes, OK, it’s going to be Mysterio at some point, surely, but before that, another enemy rearing his wet head might be none other than Hydro-Man!
Given his powers, it’s highly likely that the movie will retrofit Hydro-Man into the Elementals, perhaps replacing Hydron and thus giving the team of villains a leader with a bit more name recognition.
You don’t remember him either?
Does that mean we have to…OK, then!
movies.stackexchange.com/users/56522/Darth Locke
Who is the Water villain being shown in the  Spider-Man: Far From Home  (2019)  trailer?  Is that Hydro-Man or an Elemental?
Who is the water villain shown in Spider-Man: Far From Home trailer?
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<marvel-cinematic-universe><spider-man-far-from-home>
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movies.stackexchange.com/users/64925/Surya Tej
The water villain shown in Spider-Man: Far From Home trailer could be Hydro-Man. He  could be named  Hydro-Man  
Hydro-Man could be a reference to The Amazing Spider-Man Volume 1, Issue 212. The other villain's identity may be The Molten Man, evidenced by a number plate that refers to a Spider- man issue. "Wet Threat of Hydro-Man!" featured in the issue.