This isn't as important but would make as a great effect, once defeating the kraken (all tentacles knocked out) all of the tentacles burst out the water waving erratically before they come splashing down again.
And they float for a little while before sinking down (like the meg) and last thing heard is a final growl from the beast, that's loud enough to be heard from anyone in that region signaling to them a krakens downfall.