Can you charge underwater if you have a swim speed?

Question

Can a creature with a swim speed charge underwater?

I know some creatures specifically state they can run underwater, etc.

If a creature is bestowed a magical swim speed, can it treat water as regular terrain and thus charge while swimming?

Answer 1

Yes.

A creature with a swim speed is able to make a charge - though that creature must still meet all the necessary prerequisites of a charge. The Player's Handbook's rules for charge (p. 154-155) and the Dungeon Master's Guide rules for aquatic terrain (p. 92-93) do not suggest that charging underwater is an illegal action type.

Keep in mind the requirements for charge still apply underwater. Discuss with your DM if the "Steep Slope" requirement of hilly terrain should apply to upward (or downward!) charges in aquatic environments, but also be aware that visibility underwater is usually quite limited, especially in "Murky Waters."

Alternatively...

The DMG does allow for a character to attempt a Swim check to regain some mobility underwater. (Successful swim check yields a quarter movement speed in exchange for a move action, and half movement speed on a full action.) While it might fall just outside the scope of what the written rules explicitly state, it may be possible to perform a Swim check as part of a Charge without a Swim speed whilst underwater.

Again, discuss with your DM if being able to exchange actions for portions of your movement speed qualifies you to make an underwater charge - even slowly.

Answer 2

Charge

You must move before your attack, not after. You must move at least 10 feet (2 squares) and may move up to double your speed directly toward the designated opponent. You must have a clear path toward the opponent, and nothing can hinder your movement (such as difficult terrain or obstacles).

Assuming anything that reduces your speed qualifies for "hinder your movement", then according to the Underwater Combat table, you would be able to move without hindrance if under the effects of freedom of movement or you have a swim speed

Table: COMBAT ADJUSTMENTS UNDERWATER

 ————— Attack/Damage —————   
 Condition               Movement
 Freedom of movement     normal 
 Has a swim speed        normal
 Successful Swim check   quarter or half
 Firm footing            half
 None of the above       normal

and, according to the Swim Skill,

Make a Swim check once per round while you are in the water. Success means you may swim at up to one-half your speed (as a full-round action) or at one-quarter your speed (as a move action). If you fail by 4 or less, you make no progress through the water. If you fail by 5 or more, you go underwater. [...] A creature with a swim speed can move through water at its indicated speed without making Swim checks. [...]

Thus you can charge if you have Freedom of Movement or a swim speed and you otherwise meet all the prerequisites of a charge.

Strictly speaking, if you are just looking at the table, it looks like you could potentially charge if you are "flailing" because "none of the above" doesn't hinder movement. This appears to be an error in the table since it doesn't match the text for Swim. Fortunately, according to the rules set out in the errata, the text from the Swim skill overrides the table.

You might still make a [pedantic] case for being able to charge as you sink, as neither the text nor the table indicate it's hindered movement, but it would be absurd and likely not very useful anyway.