In between wind down work I have managed to trawl through most of what I can find on the internet from during the original trial and some of the arguments since in the vain hope that I can gather enough info to form a clear opinion. This is probably a TLDR for most but I wanted to write down my thoughts.
The character
Watson did have his character presented in a negative way that possibly overstated aspects to draw a certain picture (as all prosecutions will). However, I can't find anything that remotely negates that he was a bit of an arse. I think it is quite fair to say he wasn't the most likeable person and he definitely had some bad traits. Was a creep around women and seemed to want to paint himself as some kind of bad boy by using gang language he had picked up. The flip side is that there was no evidence of violence or previous dodgy action just talk. He thought himself a bit clever and was mouthy, even telling people he was smart enough to get away with murder.
Was he someone that could have done it? More like you couldn't say that he wasn't.
One important piece that is often overlooked is that the court heard hours of phone tap evidence between him and an ex. The public has never heard this or seen a transcript but journalists that were at the court did. One wrote that the conversations were very damning not by anything he said but by what he didn't and by the way he talked about the subject. He never once said that he didn't kill them and his attitude was more 'they've got nothing on me'.
The prosecution case
A mess. They presented so much contradictory evidence that they tangled themselves in knots and had to change their whole theory at the last minute with no evidence to support it. There were a couple of things that saved them with the jury though. The DNA evidence and the lack of an alternative. In the juries minds it would have been too coincidental that this creep in front of them had never said 'I didn't kill them' and possibly had opportunity to both do it and dispose of the bodies added to some hairs on his boat.
Most of the other so called evidence that gets picked though (sometimes quite rightly) can be stripped away and those things remain. Much of the evidence was contradictory or twisted and was a clear tactic of throwing lots of mud to create an impression of 'weight of evidence'. It's no wonder that conspiracy theorists have had a field day as so much can be disproved or heavily debated.
The timings and 'eye-witness' evidence is really unreliable. Trying to pick out solid fact from muddled memories is really difficult and so many red herrings are created. There are no definitive sightings of Watson and the missing pair together and no strong evidence that they were offloaded at his boat. There is some very rough joining of possibles that he was the person on Wallace's boat with them but also to believe that you also have to ignore what are probably the clearest recollections and timings from all of the identification and movement evidence (The worker that dropped him off at the Blade alone in conjunction with two people from boats he was rafted to that confirm the timing). The theory off him then going back to shore has no supporting evidence at all.
The Defence
Consists almost entirely of arguing against the mass of unreliable prosecution evidence and trying to put up a mystery ketch scenario which isn't supported strongly.
The outcome
I can fully understand how the jury concluded guilt. If you strip away all of the disputed stuff (mystery man, mystery ketch, identification etc) to remove reasonable doubt you are left with the basic questions of motive, means and opportunity added to 'is this person 'the type'?
Motive - he stated to people he was going there to pick up women, he spent the night unsuccessfully trying. Not that unusual in male behaviour and doesn't make him a rapist/murderer but there were also corroborated signs of him getting frustrated as the night went on and he hung around late as if waiting for opportunity.
Means - he was on a boat that could 'isolate' itself very easily
Opportunity - If the couple ended up on his boat he had a presented opportunity to isolate them. Who knows what then happened.
Was he the type? Yeah. Not a great character as shown by behaviours both before and after for long periods of time.
Then you have the lack of a viable alternative. I'm not saying that there wasn't another explanation but no one came up with any other well evidenced explanation. The police could show a lot of effort into the ketch theory including interpol, satellite picture analysis, etc etc with no results. The defence couldn't show anything strong to indicate that the police failed or didn't do their job.
Prosecution case was weak but the DNA evidence was the real nail.
Since the trial
I fully expect the DNA evidence to be debunked or have a lot of doubt placed on it. Not to the level of inadmissibility but because it is weaker and the only solid incriminating evidence presented at the trial there may be a desire to see if a re-trial finds that the prosecution case is still strong enough to convict.
On the other hand over all of this time no one has 'solved' what happened if it wasn't Watson. There is plenty of doubt thrown on parts of the prosecution case but if you remove all of that stuff and place doubt on the DNA what are you left with? That person(s) unknown had motive, means and opportunity to commit the crime but not one lead, hint or leak has since emerged
My opinion
The police work was quite single focused too early, that some of the evidence (witness identification) was manipulated, stretched or omitted but that this is noise that be removed. I would rather that they were told to make a better case and remove some of the bullshit from the start. They were under pressure to get a result and career climbing rule benders were in charge.
Time however has also been their friend. That no alternative has emerged over all of this time is quite a strong point.
Then there is one thing that although circumstantial bugs me. The fact that clearest alibi Watson could have had would have been if he didn't disappear early in the morning (at best four hours sleep after a night on the piss). I can see that he may possibly have woken and couldn't be arsed going back to sleep but so much of his other documented behaviour around people was him trying to impress them to the point of beig a piston wristed gibbon about it. He may have been a loner in some aspects but around others he was a try hard. The situation doesn't really add up.
What does add up though is him taking an opportunity to sail off and be in control over these two kids that hopped on his boat. What happened after that is anybodies guess but I suspect that it didn't go down they way everyone thinks which has increased his arrogance about the 'truth'.
All in all a clusterfuck.