Answers to some of the points raised in this thread, which has been sending a lot of traffic here.
With modest experience you can tell by the time you get between the pier extensions whether or not it is safe to go to sea. It is possible to get suckered, because in moderately rough conditions you get breaking white water to port and starboard, but the offing does not have much breaking water, especially on the flood.
On Friday that was not the case- it was a nor-nor-westerly f7, gusting 8 and an eyewitness told me that there were large breaking waves right across the harbour entrance and that the water between the extensions was very rough - you get short, steep waves bouncing off the walls coming at you from angles and clapotic chop. They would have been able to see the sea state from the outer harbour and there is room to turn but if (as seems the case) they didn’t know what they were doing they may not have had the confidence to spin the boat.
The upper harbour would have been windy, choppy but not rough. Below the swing bridge it would have been slightly rougher and by the time they were between the training walls (the inner pier arms) they would have been heaved up on metre-plus swells and should have been able to see the outer harbour and the offing. They went out at 2 hours before high water, I took this pic of conditions at high water that day:

Claimed wave heights: the estimates of 20-30 feet in the reports were exaggerated. I usually halve claimed wave heights, and on Friday I estimate 10-15 foot waves.
This what the outer harbour looks like when a big northerly is blowing right in - this is not the day of the sinking, it is the storm surge (here) three weeks before, West Pier training wall to the left, pier extension to the right. According to a witness, the conditions were not much different to this:
They appeared not to have, or be incapable of using a VHF and having asked around they asked no-one for advice before they cast off and headed downriver.