Reusable, over and over again. I can just dive in real quick ahead of the ship, set them, let them wreck it(good luck turning fast enough) then get them again once we've looted the bastards(in the name of Bahamut, of course, or whatever this setting's version of him is). If we're lucky, the immovable rods will even keep it from sinking long enough for us to grab all the gold. Or, you know, we can dive down ourselves what with magic and all to get it.
The immovable rod would punch through the hull (if the ship was moving fast enough), but it would be a small hole and take a while to set up. Overall, it's probably not worth the effort and cost to not just shoot them with cannons.
You certainly can. Whole section in the DMG for it. It'd cost 10,000 gold pieces for a +10 to UMD checks. He can make it, if nothing else.
The player cannot just make up magic items for his warlock to make. Those rules are for DMs, not players, no matter what their UMD bonus.
And if you wanted to do that, just make an Amulet of True Striking.
And UMD states nowhere that magic items can't give you a bonus to it.
Hm, could have sworn it did.
And yes, there's going to be a decently high EXP cost, but nothing like what you seem to think. Luckily for Warlocks everywhere, minimum caster level for epic spells is merely 21, and spell level is only ever considered 10th. So all you need as a warlock is +15 to UMD and you can get Epic spells, since none of them are specifically divine. +24 and you can get any and all divine spells to boot. To cost it might be a different matter, since it has a 41 DC for scrolls to cast epic spells(or possibly 32, if you go with the idea that the warlock is only caster level 12). But, that's fixed easily enough.
Scrolls cost 25 gold per caster level per spell level. Assume that the GM goes by the RAW and lets you make a scroll, counting the spell as 10th level and castable at CL 21 rather than setting higher numbers or laughing in your face at the attempted cheese. That is still 25*21*10 or 5,250 gp per scroll. This breaks down to 2675 gp and...huh, I remembered the XP costs beind higher, 210 XP. (Noticed afterwards that you calculated it. Incorrectly.) Assuming a level-equivalent encounter at level 12 with four people and only one scroll for each, the warlock is throwing away close to a third of his XP just to cast epic-level spells. In addition, the treasure doled out is typically around 9800 gold; divide by four, and the warlock only gets 2450 gold.
He's spending more treasure than he's getting on this tactic!Come Level 15, and these encounters cost "merely" a fifth of his XP and half of his treasure. Level 20? He's still spending a seventh of his XP and a quarter of his treasure.
A quarter of his Level 20 treasure! That's nearly the price of a +5 vorpal axe or a Ring of Elemental Command, greater than an Iron Flask or one of those +5 Ability books. And assuming he used one such scroll per encounter, 13 encounters per level, he would have spent nearly 22,000 XP, over a tenth of his total. This ain't cheap.
Plus, hire a bunch of commoners, give them half decent weapons, give them a potion and tell them it will make them super-strong and skilled and they can keep a quarter of the dragon's treasure for themselves. Tell them to drink it only once you've left(bullshit a reason; you'll beat their sense motive check easy). It turns out to be a potion of Lord of Nightmares. Best part is, they'll probably even survive, if the dragon doesn't. And the dream larva doesn't suspend himself over lava or something.
Several problems with this, starting with the fact that potions can't be made with higher than 3rd level spells.
Edit: I'd like to see Kre!aqil take me on in an aerial fight. 10 gold pieces says I can fly faster than you.
10 gold says that it doesn't matter, because A. water provides cover and B. your scenario involves swimming around underwater, not flying 15 feet above it.
