So my group following the events of this post did find time to head out to Kraken's Cove before the ball at Castle Teraknian. Once there, they found the place mostly empty with a couple of Crimson Fleet lackeys 'on guard'.

In an effort to keep diplomatic ties open, they explained that they understood that Vanthus was planning on stealing from the fleet through Brissa, and all they wanted was to take Vanthus back to Sasserine to pay for his crimes. They had no problems with the fleet itself, they purely wanted Vanthus

Vanthus or the fleet didn't show before the group had to return to the ball, so the group took the lackeys back to Sasserine, set them free in the city (as a show of good faith to the fleet) and headed to the ball.

Now, I've got a whole different sub-plot going on with the ball right now, but once the group is done with that, they will go back to Kraken Cove. So, while the party were at the ball, one of the lackeys made it to the Fleets emmisary in Sasserine and told him everything that happened. I'm going to have the emmisary get a message to the fleet that Vanthus isn't to be trusted, so the events at the cove (prior to party arrival) are slightly different.

The idea I have is that Vanthus turns up with Brissa with the intention of springing the trap as planned, yet to make sure it goes well has drank a potion of Glibness. The Fleet (Harliss Javell) orders him to be taken prisoner, but Vanthus manages to talk himself out of it long enough for the trap to be sprung. During the initial choas, Javell pursues Vanthus into the Slippery Eel where they accidently set off the Savage Tide. Vanthus managed to flee, and the events of the cove unfold as described

As the party knows the layout of the cove, I'm going to have to make some slight alterations, for example, the party weakened the walkway in their initial visit, so that will have collapsed (most likely impailing a unlucky pirate in the process)

One thing I've not planned out as yet is how does Vanthus escape? He can't head back to the city, he can't escape via the back of the cove (my PC's are likely to come in that way, and if they do, I'm going to move Harliss to a different room) so where does he go? I'm tempted to have him escape with a couple of the mercenaries he hired using a Folding Boat (we're using PF rules) and head east (to where he gets picked up by the Brine Harlot and taken to Scuttlecove)

Does that seem believable? Any advice would be handy!

Or he could escape via the water. He can have some allies on a boat outside Kraken's cove, so that he can swim there. Perhaps you could give him a potion of swimming or a potion of water breathing.

I like your idea, since it involves the PCs more in the Crimson Fleet plot.
But before you use this plot, please remember that Vanthus will be a member of the Crimson Fleet again when they attack Farshore. Though even though he is not trusted now, he turns up later as a member (and then even has the Lemorian template, so he will be a half-demon). So perhaps you should think about what you intend to do with that. Perhaps the Crimson Fleet is only marginally involved with the demonic plot now and later gets deeper into it, so that Vanthus can consolidate his position when the demonic members gain more power. It would give Harliss a better reason for leaving as well.
You could also consider giving the Crimson Fleet two factions, of which one cooperates with the demons.

Thanks Luna, I'm probably going to have him escape with some unnamed allies via a folding boat. He'll probably want to return to Sasserine, but get overruled and sail east, where he'll end up brought in by the Fleet.

I'm pretty much going to have the Fleet split into two factions, the far and away larger faction being headed up by Wyther, who will be the one's who are being controlled (like it or not) by the demon connection. The other will end up being Harliss and the crew loyal to her, most likely to be only a handful of people. At the moment (during BWG) Harliss hasn't broken away from the Fleet, but is starting to have her doubts about what the Fleet are upto.

Her job in BWG would have been to bring the shadow pearl from Wyther (under his command) to Kraken's Cove to pass it onto a smuggler (Penkus) who'll take it to Sasserine. Not only is Penkus already dead, the tide gets triggered anyway, so she failed all round. Not her fault, but Wyther isn't going to see it that way when she returns to Scuttlecove. All the more reason for her to look to break away from the Fleet.

As for Vanthus, I'm going to follow the background from SoS to why he actually ended up as a captian in the Fleet (i.e. Ghorvash). Even though the Fleet know Vanthus isn't to be trusted, they know full well that going against Demogorgons wishes would be a 'very bad idea'.

Nice! It gives you the extra possibility to have a nice resistance group led by Harliss. If the players ever mess up (e.g. later in Farshore), she could even rescue them or help out - and they won't hear the end of it of course!

I am curious to hear how it goes.

Totally going to steal the faction idea. That would also explain why most of the fleet is absent from the Wreck

Just to let you know how this went... it's been a while.

The actual event in Kraken's Cove went as I'd planned. The party turned up to find the chaos, battled a load of savage creatures, met Javell (with mixed success) and hightailed it back to Sasserine to save Lavinia.

They saved Lavinia and we finished BWG. Unfortunately, that's where our group fell apart :(

The good news is that I started ST again with another group and they're loving it so far, even to the point where I've managed to get the survivors from my first group moved over :)

They're at Kraken's Cove right now, just about to meet Javell (however they're going to meet her on ship as they took too long at the cove so she escapted). As it seems like such a good plotline, I'm going to play the Crimson Fleets story in exactly the same way.

Sad that your group fell apart. Hope you can continue to play the campaign all the way with the new group.
Mine will be starting Serpents of Scuttlecove next week (which is where Harliss returns).