Zombie Pirates without a ghost ship? What next? No hooks? No eye patches? No parrots? Without units like this, it's just a fancy zombie list.
By reforming the tentacles, the kraken would be able to either attack the entire front rank in a broad unit or to claim a rank bonus since, with the tentacles, it behaves more like a unit than a single monster. I also think that the army should have access to the necromancy spells as they are the fundamental building blocks of raising the dead.
Typically, the siren is solitary in the various folklore; ironically, the only exception to that I can think of is in the Odyssey itself (and thus, O Brother Where Art Thou.) She'd also fill the gap of a lord level magic user and fill in the archetype of a sea witch, calling down storms and curses. Otherwise, why not just have wraiths? Additionally, the forced movement ability is more in keeping with a character than a unit. For the 40K players, this is comparable to the Lash of Submission Slaanesh power, which proved fairly unpopular. To then give it to a unit, might be a bit rich for the opponent.