Last night I underestimated what the powers of a simple zombie template could do to a cachalot whale and my party nearly died (had I not fudged a lot of its stats down halfway through the battle). These are level five characters fighting from a ship that still miraculously floats. But once converted, the whale had an AC of 27. I didn't convert its attack bonuses, but I believe thye'd be higher than its living counterpart, whihc is a +17 to hit with its bite attack for 4d6+12 damage. The bastard had 24 HD, all d12s. Sadly, the party has only a bard for an arcane spellcaster and the cleric was absent.
Just a word of warning for anyone who thinks a little zombie template is just a way to spice the game up.