He could have dumped the body somewhere else and only sank the sub to wash away the evidence. The sub was missing for over 12 hours. Who knows how far that thing can get in that time.
Here's hoping nothing happened and the journalist just took a nap at a hostel or something, but the optics are quite bad, it's a lot of coincidences which look pretty nasty.
He could just claim that they both abandoned the submarine and then got separated. What benefits are there to make a elaborated plan?
http://www.bbc.com/news/world-europe-40916787
He claimed earlier that she was dropped off many hours prior to the incident. He apparently has a new and different story now, though the police aren't saying what it is.
He can't claim they both abandoned the submarine when it sunk, as there were eyewitnesses to that event.
What is likely, given what's known at this point? I don't see some more likely scenario. Her body wasn't on the submarine, and the eyewitness account of the sinking supports the idea of a deliberate scuttle. The police say it looks deliberate as well.
Ships aren't build with built-in scuttle capabilities, military submarines can be scuttled because you can blow yourself up.
The likely scenario is that the submarine has failed, she either died before she could escape or the more likely scenario is that Madsen panicked, saved himself and left her behind to die.
[1]http://www.telegraph.co.uk/news/worldnews/europe/eu/10538172...
They were looking for the submarine for hours before it showed up and sank in front of eyewitnesses. It was in the news already at that stage. I think he may have panicked when he learnt that everybody was looking for him and the woman and then sank the sub. One eyewitness saw him in the tower, then he went down into the sub, then back up, and then it sank. My guess would be that he went down and pushed the submerge button, went back up and got himself "rescued" by a passing boat.
Submarines are. Ballast tanks plus leaving a hatch open.
Also, the sub didn't sink until 11am the day after. She did not plan on an overnight trip in the sub. That's why I think there's more to this story than an accidental sinking.
The vast majority of small boats have through hole fittings for things like the heads (toilet) or cooling water intake for the engine. Opening the valve and smashing the pipe off is a trivial way to sink the boat. Larger ships will have similar fittings though the sinking will take longer. As noted by your others there's ways to intentionally flood a submarine too even if they're not intended for that purpose.
Guess it depends upon you definition of "Small Boat". Certainly your standard mono hulled yacht will sink rapidly if you fill it with water (which is purposefully done on occasion, if sailing across an ocean and major damage forces you to abandon ship to another vessel the insurance company may ask you to sink the boat. It's not worth the costs required to salvage and is a danger shipping if left afloat).
that makes this quite unlikely