Brief: This oceanic dinosaur is a terror of for both sea dwelling humanoid and fisherman. Its flesh is typically a light gray or very pale green backside with a whiter belly. It is capable of emerging on land, albeit a very short distance, beach goers are advised to move in-land as quickly as possible. Rumors of magical Liopleurodon's persist while many a person really, really hopes that these things never acquire magical powers.
Attributes: Agility: d8, Smarts d4 (A), Spirit d8, Strength d12+5, Vigor d12
Skills: Fighting d10, Swimming d12
Traits: Pace: 3 (Land) / 10 (Aquatic); Parry: 7; Toughness: 13; Size: 5
Special Traits: Hardy: Cannot be wounded on a second Shaken score; Large: Attackers add +2 to their attacks due to size; Aquatic.
Bite: Str+d8; S: Reach 1 due to long neck.
Swords & Wizardry (And other rules-lite games)
Hit Dice: 16
Armor Class: 6 
Attacks: Bite (2d12); Flipper Slap (1d10)
Saving Throw: 3
Special: Swallow Whole; Aquatic
Movement: 3/20 [Swimming]
-Swallow Whole: If a Liopleurodon hits a target 4+ higher than necessary the target is swallowed whole by the Liopleurodon. The victim can test to draw a dagger (Test Dexterity with penalty of -2 or -10% depending on attribute check system) and stab their way out. The AC for cutting their way out remains at 6  but they gain a +1 to hit due to proximity. If they can manage to do 10 or more points of damage they are regurgitated at a random target.
-Aquatic: This creature can breath underwater and swim impeccably. It has some small amount of movement on land and can remain on land for an hour before returning to the sea.
"We Have to Close the Beaches!"
Brief: A popular fantastical tourist destination is being plagued by mysterious deaths in the water. The party has various ways of finding out (observation, contacting friendly sea humanoids, magical means). They learn that a group of angry or malicious sahuagin have sicked a rogued liopleurodon on the community. The party has a couple of ways of dealing with such a creature - lure the beast onto land, track him to his cave, or lure him to the coral reef base the sahuagin are using. Better do it before the local celebration draws plenty of victims to the shores.
"A Magical Liopleurodon!"
Brief: Well, it happened, one of these monsters have gained either magical powers or can breath energy. Sea captains have pooled together with pirates to stop this creature and several attempts have failed. Now the amateurs turn to adventurers to deal with the problem.
Brief: The party's patrons is an eccentric who has managed to maintain an aquarium with large viewing panes in his sprawling manor (that is partially submerged and protected via magical runes and fine Gnomish construction). Through means esoteric and mundane, he has managed to build a new viewing gallery with an empty pen. He wants to put a liopleurodon in it and wants to hire the party to bring back a live one. Break out the poisoned darts, spears, and arrows and bring the towing line.