Ichthyosaur (CR 6)

Large Animal
Alignment: Always neutral
Initiative: +3 (Dex); Senses: low-light vision, scent, Listen +9, and Spot +10

AC: 18 (-1 size, +3 Dex, +6 natural), touch 12, flat-footed 12
Hit Dice: 10d8+50 (95 hp)
Fort +14, Ref +10, Will +4
Speed: Swim 50 ft.
Space: 10 ft./5 ft.
Base Attack +7; Grapple +16
Attack: Bite +11 melee
Full Attack: Bite +11 melee
Damage: Bite 1d8+7
Special Attacks/Actions: -
Abilities: Str 20, Dex 17, Con 20, Int 2, Wis 13, Cha 9
Special Qualities: Hold breath
Feats: Alertness; Combat Reflexes; Great Fortitude; Swim-By Attack
Skills: Listen +9, Spot +10, and Swim +13
Advancement: 11-15 HD (Large); 16-30 HD (Huge)
Climate/Terrain: Warm aquatic
Organization: Solitary, pair, or school (3-6)
Treasure/Possessions: None

Source: Stormwrack

Hold Breath (Ex): An ichthyosaur can hold its breath for a number of rounds equal to 6 x its Constitution score before it risks drowning. For a typical ichthyosaur, this is 120 rounds, or 12 minutes.

Skills: An ichthyosaur 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.

An ichthyosaur's jaws are designed for catching small fish with ease. Against a larger foe, such as a swimming adventurer, it uses hit-and-run tactics, racing in to snap and slash with its long, fang-studded jaws.