Our party needs to go underwater. Our party also includes a warrior who is heavily specialized into a pair of +1 keen gold great scimitars, which deal slashing damage and thus take substantial penalties underwater. Because of their specialization, switching to another weapon also amounts to a substantial penalty. So either option is going to make the character have limited ability to contribute, and likely make the player not enjoy the session. We’re trying to brainstorm the best way to prevent that.
We do not have an opportunity to select new feats, or take levels in other classes, before we need to do this. We are 10th level, but most of us do not have anything to particularly contribute (a 10th-level crusader, a 10th-level beguiler, a multiclass character with the manifesting of a 6th-level egoist but powers tailored to self-buffs and battlefield control).
We do have a character with the spellcasting of a 9th-level wizard and something of a specialty in scrolls. They’re specializing in familiars, with Improved Familiar and four levels in the spell sovereign prestige class, and the changeling wizard racial substitution level from Races of Eberron, which allows their familiar to switch to any other kind of familiar as a full-round action, including into a living spell of 1st or 2nd level, so we potentially have access to a lot of spell and spell-like abilities that way.
We also have numerous NPC allies who could provide access to other spells that might be scribed with the wizard’s assistance, or even added to the wizard’s spellbook. There might plausibly be magic items we could borrow (we have a lich, a dragon, and a fey queen among our allies), and there’s an NPC artificer who might be able to create something in the time we have allotted (maybe three days), if it’s cheap enough.
Note that changing these weapons to another type—which we considered—doesn’t work because heavy-metal weapons require Exotic Weapon Proficiency in precisely the type of weapon, so the warrior wouldn’t be proficient with, say, +1 keen gold rapiers. Changing the +1 keen gold great scimitars to deal piercing damage—without making them anything other than great scimitars—might work, as would anything that says you can use non-piercing weapons underwater without penalty.
Freedom of movement, a 4th-level spell for most classes, does the latter explicitly. It has a range of touch, and lasts 10 minutes/level, so it’s plausible that the wizard could cast this on the warrior, but we don’t know how long we’ll be under. For all we know, the entire dungeon may be underwater the entire time. We have water breathing, so the wizard could re-cast freedom of movement, but that is dedicating a lot of the wizard’s second-highest-level spell slots, plus it’s possible for the wizard and warrior to get separated.
Heart of water is a 3rd-level spell for a few classes—including wizard—that can be expended to get a 1 round/level freedom of movement effect, but it’s self-only.
But the key thing to me is, the ability to use weapons underwater is a tiny fraction of freedom of movement’s power. Is there any spell that does just that? Ideally for a longer duration, and at a lower spell level? Any cheap magic item?
Commission the item now, and it'll be done just in time
The armor augment crystal greater crystal of aquatic action (Magic Item Compendium 25) (3,000 gp; 0 lbs.) does what's needed and much, much more: it negates armor check penalties on Swim checks, grants a swim speed, and the bearer suffers no movement or combat penalties underwater in a way that's explicitly like the spell freedom of movement. Almost as an afterthought, it also grants the ability to breathe underwater.
In traditional campaigns that I've DMed, mundane warriors buy this crystal even when they aren't expecting aquatic adventures because just having it available opens vistas for exploration. Seriously, I'd recommend the scimitar wielder get one even if the warrior weren't currently planning to beat up sea monsters.
If the warrior's armor is insufficient to bear such a crystal (the armor needing at least a +3 magical enhancement bonus), there's the more expensive and less versatile shark tooth amulet (9,000 gp; 0 lbs.). It does pretty much nothing but make it so a wearer's conventional attacks can be launched more or less normally underwater, and it occupies the precious neck slot. Nonetheless, if this is a better option, a shark tooth amulet is created with the help of an undine, and at least one of a lich, dragon, or fey queen should know an undine who might be able to hook up an allied warrior.