So I've really wanted to combine the Watcher in the water with the corsairs for a while now, and even though the two forces could never meet I still feel that the army would have a nice theme, hence why I am starting this topic to discuss how I should go about building the force. I picked up 24 corsairs on ebay and am gonna get everything else I need ocne I have ironed out a solid list. Here is the army I have in mind:

The Watcher
Hasharin, Corsair Captain, and Harad taskmaster collectively leading:
12 Reavers, 11 Corsairs with spears, 6 Corsairs with bow, 4 Corsairs with spear and bow.

With 38 models it isn't the most impressive bodycount, but the watcher's deployment/tentacle rules mean that dividing and conquering the enemy will be a manageable task against any force. However I could drop the hasharin for another corsair captain and give all the soldiers two pieces of kit, meaning I have a very versatile rag tag buncha renegades, but lose out on the Hasharin's killing power/special rules and survivability.

Either way, this is something I wanna do well, so if you have any tips or suggestions as to what I should get please lemme know

Sounds like a great idea thematically - a Kraken of the deep being summoned up to fight with the Corsairs. Makes me want to play a naval game actually, where The Watcher would be a great monster to play.
I can't comment on it from a gaming point of view, but I'd love to see the force painted up.
Have you considered Dalamyr and a Bo'sun instead of the Hasharin and taskmaster, in order to keep it pure Corsairs?

Yes I have considered Dalamyr but my best friend who hits tournaments with me's favourite model is Dalamyr - and since the mega plan is to combine our forces in doubles i think I'll avoid Dalamyr. Besides if the leader of the Hasharin is from Umbar it is safe to assume that all of the assassins have the option to work with the corsairs. I guess that leaves the taskmaster as the 'unthematic' guy but I'm sure with a bit of conversion work he will look piratey enough

From a gaming perspective would people recommend a bo'sun over a taskmaster? I'm thinking the taskmaster will be necessary to bring an extra 2 might and the potential for making the armies 6 might last a little longer.

I have to say that the combination provides a great theme and sounds great to play against. Have you converted any reavers yet, and if so can we see a picture?

No Reavers yet, two 'pike'men painted, struggling with how to base them though :/

I would base them on some wooden planks with somethimg like a barrel for decoration for one or two.