The diving suits worn for diving at a depth of 25000 feet are normal diving wet suits. Diving at such a depth is completely impossible in such a suit. Not to mention that the diver should undergo a very lengthy compression / decompression cycle.
The Siren II was descending to a depth of 45K feet when it was attacked. The deepest point on Earth is the Mariana Trench in the Pacific Ocean which is about 36K feet. This has been confirmed by satellite imaging, sonar and topography maps.
When a seaweed sample is sent in, it is pulled up through an open hatch on the deck. An open hatch at that depth would have instantly flooded the sub.
the landing gear of the Siren 2 is so thin, it can impossibly hold this weight.
When Wick asks what happened to the Siren 1, Steensland tells him that the nuclear power system of the Siren 1 experienced a complete failure. Clearly a submarine of this size can impossibly contain a nuclear power system. Neither is there a need to have such a ship powered by a nuclear reactor.
The shots of the Siren 2 when cruising on the surface of the ocean reveal that this is not the Siren 2. The top shape of the submarine is different from the siren.
When the captain orders a test dive of the submarine, the shots before that of the submarine show it already submerged.
When a diver makes a sortie, the crew can follow him on television screens in the submarine. This is clearly completely impossible as there are no outside cameras around the diver to follow him.
The closeup screens of Sven's dive show depth in feet, and when there is a switch to the view of the screen together with the operator, it clearly marks depth in meters.
The operator's screen shows a latitude of 103° 43' 35''. Not only is this missing north or south indication, but more importantly, latitudes range between 90° and 90° south. So these coordinates are impossible.
The computer screen showing the dive of Sven (with a type on the screen calling him Sven driver in stead of diver) shows the depth of Siren 1 to be at 26190 feet, but the location of the dot is below 23550 feet.