Dire Barracuda (CR 4)

Large Animal (Aquatic)
Alignment: Always neutral
Initiative: +2 (Dex); Senses: low-light vision, scent, Listen +6, and Spot +6

AC: 17 (-1 size, +2 Dex, +6 natural), touch 11, flat-footed 15
Hit Dice: 8d8+16 (52 hp)
Fort +8, Ref +8, Will +5
Speed: Swim 80 ft.
Space: 10 ft./5 ft.
Base Attack +6; Grapple +14
Attack: Bite +9 melee
Full Attack: Bite +9 melee
Damage: Bite 1d8+6
Special Attacks/Actions: Sprint
Abilities: Str 19, Dex 15, Con 14, Int 1, Wis 12, Cha 2
Special Qualities:
Feats: Alertness; Iron Will; Run
Skills: Jump +29, Listen +6, Spot +6, and Swim +12
Advancement: 9-12 HD (Large); 13-24 HD (Huge)
Climate/Terrain: Warm aquatic
Organization: Solitary, pair, or school (3-6)
Treasure/Possessions: None

Source: Stormwrack

Sprint (Ex): A dire barracuda can move up to three times its normal speed (240 feet) when it makes a charge.

Skills: A dire barracuda has a +8 racial bonus on any Swim check to perform a special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.

The dire barracuda is much more aggressive than its smaller cousin and sometimes hurls itself out of the water to attack.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.