How is this:
Giant Turtle
Size/Type: Huge Animal
Hit Dice: 10d8+60 (105 hp)
Initiative: +0
Speed: 20 ft. (4 squares), swim 20 ft.
Armor Class: 23 (-2 size, +15 natural), touch 8, flat-footed 23
Base Attack/Grapple: +7/+22
Attack: bite +7 melee (2d6+3)
Full Attack: bite +7 melee (2d6+3)
Space/Reach: 15 ft./10 ft.
Special Attacks: capsize
Special Qualities: Hold breath, Low-light vision
Saves: Fort +13, Ref +7, Will +6
Abilities: Str 25, Dex 10, Con 21, Int 2, Wis 12, Cha 2
Skills: Hide 8*, Listen 6, Spot 6, Swim 15
Feats: Alertness, Great Fortitude, Iron Will, Endurance
Environment: Temperate aquatic or marshes
Organization: Solitary or bale (6-11)
Challenge Rating: 4
Advancement: 11-21 HD (Huge), 21-30 HD (Gargantuan)
Level Adjustment: —
Capsize (Ex)
A submerged giant turtle that surfaces under a boat or ship less than 20 feet long capsizes the vessel 90% of the time. It has a 30% chance to capsize a vessel from 20 to 60 feet long.
Skills
A giant turtle has a +8 racial bonus on any Swim check to perform some 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.
*Giant turtles have a +8 racial bonus on Hide checks when submerged.
-Stuart