Shark, Large (CR 2)

Large Animal (Aquatic)
Alignment: Always neutral
Initiative: +6 (+2 Dex, +4 Improved Inititive); Senses: blindsense, keen scent, Listen +8, and Spot +7

AC: 15 (-1 size, +2 Dex, +4 natural), touch 11, flat-footed 13
Hit Dice: 7d8+7 (38 hp)
Fort +6, Ref +7, Will +3
Speed: Swim 60 ft.
Space: 10 ft./5 ft.
Base Attack +5; Grapple +12
Attack: Bite +7 melee
Full Attack: Bite +7 melee
Damage: Bite 1d8+4
Special Attacks/Actions: -
Abilities: Str 17, Dex 15, Con 13, Int 1, Wis 12, Cha 2
Special Qualities:
Feats: Alertness; Great Fortitude; Improved Initiative
Skills: Listen +8, Spot +7, and Swim +11
Advancement: 8-9 HD (Large)
Climate/Terrain: Cold aquatic
Organization: Solitary, school (2-5), or pack (6-11)
Treasure/Possessions: none

Source: Monster Manual

Keen Scent (Ex): A shark can notice creatures by scent in a 180-foot radius and detect blood in the water at ranges of up to a mile.

Sharks circle and observe potential prey then dart in and bite with their powerful jaws.

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.