Not very encouraging. But thanks for the ideas. One possible problem: the DM has ruled that lightning bolts do 1/4 damage underwater, and that the damage is a globe that originates where the bolt originates: the caster's fingertips or wand tip.
Here's what I've been thinking:
Buff up fighters (including abjurer) with buff spells, haste, invis. Abjurer also gets Stoneskin and then casts Vampiric touch and holds charge.
Melee attack like mad, but stick together. The kraken's reach is bad news, but I'm perfectly happy hewing at tentacles as I am at hewing the body--the kraken bleeds just the same.
Sorceress stands off at range and magic missiles. Over and over again.
When abjurer is finally hit and grappled, the Vampiric touch spell will go off. That's another 8d6 of dam to the Kraken, which has the bonus of adding to the abjurer's hp.
To get out of grapple, Abjurer casts Dimension Door (only a verbal component) or uses Wand of Dimension Door.
Abjurer can use Dimension Door (and haste) to appear next to grappled friend, then touch friend and use another DD to pop away.
This just might let us take the damn thing out.
But I also like the idea of forgetting about the kraken for awhile and instead exploring the cave. Maybe there'll be a way to trap it or crush it under rock.
Any other ideas?