Can you take the run action underwater?

Question

I have an underwater encounter coming up soon in my D&D 3.5e game. They will be entering a flooded section of dungeon. Rules say PCs can walk on the bottom, at half normal speed.

(DMG, p. 92, or SRD):

Condition: Firm footing. Movement: Half

Creatures have firm footing when walking along the bottom, braced against a ship’s hull, or the like. A creature can only walk along the bottom if it wears or carries enough gear to weigh itself down—at least 16 pounds for Medium creatures, twice that for each size category larger than Medium, and half that for each size category smaller than Medium.

Can they run?

Answer 1

No, you can't, your movement is Hampered.

Normally, whenever you cannot move your normal speed without reductions to your speed, your movement is said to be Hampered, and thus you cannot run or charge.

Hampered Movement

Difficult terrain, obstacles, or poor visibility can hamper movement. When movement is hampered, each square moved into usually counts as two squares, effectively reducing the distance that a character can cover in a move.

You can’t run or charge through any square that would hamper your movement.

One may argue that your movement was reduced in half, but not hampered. The glossary entry, however, says this:

When restricted to moving at half speed, count each square moved into as 2 squares, and every square of diagonal movement as 3 squares. If you are restricted to half speed, you can't run or charge, nor can you take a 5-foot step.

This is how Skip Williams defined moving at half speed (which you can take with a grain of salt):

Half Speed: Some conditions, such as blindness or entanglement, force a creature to move at half speed. A creature reduced to half speed always moves as though its movement is hampered. (Each square costs 10 feet of movement to enter, and each diagonal costs 15 feet.) Creatures reduced to half speed cannot charge, run, or take a 5-foot step.

You can still make double moves, use two move actions in a round.

Answer 2

Only if you are a creature with an innate "swim speed" http://www.d20srd.org/srd/skills/swim.htm

A creature with a swim speed can move through water at its indicated speed without making Swim checks. It gains a +8 racial bonus on any Swim check to perform a special action or avoid a hazard. The creature always can choose to take 10 on a Swim check, even if distracted or endangered when swimming. Such a creature can use the run action while swimming, provided that it swims in a straight line.

Answer 3

If ruling strictly by the book then it's a matter of what "the book" means for your group. I'd recommend exercising DM adjudication to allow or disallow running as seems appropriate for the specific narrative context.

Going by the DMG, applying the effects of difficult terrain is a GM call in almost every case because otherwise the concept would almost never be used. This seems like a reasonable bit of adjudication. If we include the Glossary, however, that situation changes dramatically.

Aquatic terrain --indeed pretty much any terrain type that isn't undergrowth-- isn't called out as difficult or hampering (which would prevent running) or inherently having any other terrain keyword. Halving one's movement (as well as other more exotic effects like reducing damage!) is presented as a separate effect. Several other terrain types (like marshes) also do this by extracting parts of the "hampered" text to use without explicitly saying that means all of the other effects of hamperedness apply. By the DMG, then, it's a DM call whether to apply the (extremely broad) concepts of "hampered" or "difficult" terrain to any given situation (except, again, undergrowth).

If you abide by this DMG-focused reading, "movement half" on its own means: the character's base move speed (usually 30) is halved (15). Walking is moving at base speed, running is moving four times your speed, and so forth, as normal--the character is just using the adjusted base movement number so everything's half as fast.

This changes when we bring in the glossary, which redefines its terms as descriptive rather than prescriptive: that is, the full effects of "hampering" and "difficult terrain" are invoked by the narrative of encountering just part of their effects. This moves DM adjudication in the opposite direction, applying hampered to any movement-halving terrain unless/until the GM declares an exception.

So if we go strictly by the rules, either almost nothing prevents running or almost everything prevents running depending on the set of rules we're giving precedence.

Personally I think it's silly to enforce such a blanket decree either way. That sort of thing makes sense in a system with narrow, bounded sets of conditions (like 4e) but 3.5's virtue lies in its extremely open approach to exception-based subsystems. Being able to separate base movement changes from movement action changes seems more coherent for the 3.5 paradigm of having lots of fiddly bits to play with.