This is for Pathfinder and has the higher CR that you are looking for.
Gargoyle, Sharkacinth CR 5
CE Large Magical Beast (aquatic)
Init +5; Senses darkvision 60 ft.; low-light vision, keen scent, Perception +5
AC 17, touch 11, flat-footed 16 (+1 Dex, +6 natural)
hp 57 (6d10+24)
Fort +9, Ref +6, Will +0
Swim 60 ft.
Melee Bite +11 (2d6+6) (grab), tail slap + 11 (1d8+6)
Str 23, Dex 12, Con 18, Int 6, Wis 11, Cha 7
Base Atk +6; CMB +13 (+17 grapple); CMD 14
Feats Improved Initiative, Skill Focus (swim), Swim-By Attack
Skills Swim +12, Perception +5, Stealth +9 (+15 in stony areas); Racial Modifiers +2 Stealth (+6 in stony environs)
Languages Common, Aquan
Environment any aquatic
Organization solitary, pair, or shiver (3–12)
Freeze (Ex) A gargoyle can hold itself so still it appears to be a statue. A gargoyle that uses freeze can take 20 on its Stealth check to hide in plain sight as a stone statue.
I'm not as good with Pathfinder as I am with 3.5 so you should double check this stat block. You can advance this by HD and by size. At 12 HD it could be Huge. Also, here is a link to Swim-By Attack feat: http://www.d20pfsrd.com/feats/3rd-pa...swim-by-attack Dspeyer had a good idea of adding a tail slap so I amended this to add that too even though sharks don't have a tail slap.