Something that can go differently then as written as it did for me.
1 the party killing the hurricane king in the counsel meeting. (my party did that at start of book 5 technically, killing him before he could do more then one move action. ) so no need for the dungeon crawl.
2 scry and die on the fleet. if you let the party know the name fo the admiral leading it and/or the name of her bodyguard it is a simple thing scry and find out when she sleeps, go invisible teleport and kill bodyguard then her. using cloak of dreams to make bodyguard fall asleep. then go around with overland flight with inadvisability and grabbing something from the other captains to scry on them for next time. while also planting as many castings of fire seeds using holly berry's inside of the ship at night. thus killing a fair amount of the non combat crew and setting the ship on fire from the inside. thus a party of 4 can take on a armada of 50 or so ships and win.( well a high level caster can do this by themselves to any military force not prepared for it. ie scry, fly, greater invisibility, teleport, evocations of some kind, teleport out, do that a few days and watch common military forces die)
thus the last book can turn into one short session with no need for a fleet battle. in fairness yes i know that devil has see invisibility but there are only 4 of them and i missed that in there stat block. You can take this as a warning of how to avoid this or as a easy way to resolve the book as you will.
another idea/warning only the hurricane king has a Amulet of Proof against Detection and Location in the book, so scry and die is something you can do on his first officer who the party has seen before. risk would be things could attack from behind.