Dire Eel (CR 3)

Large Animal (Aquatic)
Alignment: Always neutral
Initiative: +5; Senses: low-light vision, scent, Listen +5, and Spot +6

AC: 13 (-1 size, +1 Dex, +3 natural), touch 10, flat-footed 12
Hit Dice: 6d8+15 (42 hp)
Fort +7, Ref +6, Will +3
Speed: Swim 30 ft.
Space: 10 ft./5 ft.
Base Attack +4; Grapple +12
Attack: Bite +8 melee
Full Attack: Bite +8 melee
Damage: Bite 1d8+6
Special Attacks/Actions: Attach
Abilities: Str 19, Dex 13, Con 14, Int 1, Wis 12, Cha 2
Special Qualities:
Feats: Improved Initiative; Toughness; Weapon Focus (bite)
Skills: Hide +1 (+5 in coral, Listen +5, Spot +6, Swim +12, or kelp), and rock
Advancement: 7-12 HD (Large); 13-18 HD (Huge)
Climate/Terrain: Warm aquatic
Organization: Solitary or pair
Treasure/Possessions: None

Source: Stormwrack

Attach (Ex): If a dire eel hits with its bite attack, it uses its powerful jaws to latch onto the opponent's body and automatically deals bite damage each round it remains attached. An attached eel loses its Dexterity bonus to AC and has an AC of 12.

An attached eel can be struck with a weapon or grappled itself. To remove an attached eel through grappling, the opponent must achieve a pin against the creature.

Skills: Dire eels have a +4 racial bonus on Hide checks, which improves to +8 in areas of coral, rock, or heavy kelp. An eel 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.

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.