The following material is Open Game Content from the Menace Manual SRD
A sea serpent has a snakelike body between 30 and 200 feet long. Other features may vary with the individual creature— some sea serpents have slender fins like those of an eel, while others have completely smooth bodies. The head of one may be shaped like a horse, another like a seal.
Hazy Outline (Su): Part of the reason that so few sea serpents have ever been spotted or photographed is that they can make their forms indistinct and thereby blend into the lapping waves. As a full-round action, a sea serpent can blur its outline for a number of rounds equal to its Hit Dice. This distortion grants the sea serpent one-half concealment (20% miss chance). A sea serpent cannot be recorded on any visual media while in this state—a photograph shows only a fuzzy haze. A see invisibility spell does not counteract this effect, but a true seeing spell does.
Improved Grab (Ex): To use this ability, the sea serpent must hit an opponent at least one size category smaller than itself with its bite attack. If it gets a hold, it automatically deals bite damage each round that the hold is maintained, and it can swallow in the next round.
Skill Bonuses: The sea serpent’s coloration and shape grant it a +10 species bonus on Hide checks made while it is in water.
Swallow Whole (Ex): If a sea serpent begins its turn with an opponent two or more size categories smaller than itself held in its mouth, it can attempt a new grapple check as though trying to pin the opponent. If it succeeds, it swallows its opponent, automatically dealing bite damage. Once inside the sea serpent, the opponent takes bludgeoning damage equal to the serpent’s tail slap attack plus 1d6 points of acid damage per round from the monster’s gullet. A successful grapple check allows the swallowed creature to climb out of the gullet and return to the sea serpent’s maw, where another successful grapple check is needed to get free. Alternatively, the swallowed creature can attack from inside the gullet using claws or a Small or Tiny slashing weapon. Dealing at least 20 points of damage to the gullet (Defense 25) in this way creates an opening large enough to permit escape. Once a single swallowed creature exits, muscular action closes the hole; thus, another swallowed creature must cut its own way out. A Huge sea serpent’s gullet can hold 2 Medium-size, 8 Small, 32 Tiny, or 128 Diminutive or smaller opponents.
Sea Serpent: CR 7; Huge magical beast; HD 9d10+27; hp 76; Mas 17; Init +7; Spd swim 60 ft.; Defense 15, touch 11, flat-footed 12 (–2 size, +3 Dex, +4 natural); BAB +9; Grap +25; Atk +15 melee (2d6+8, tail slap); Full Atk +15 melee (2d6+8, bite or tail slap); FS 15 ft. by 15 ft. (coiled); Reach 10 ft.; SQ darkvision 60 ft., hazy outline, improved grab, low-light vision, swallow whole; AL none; SV Fort +9, Ref +9, Will +5; AP 0; Rep +0; Str 26, Dex 16, Con 17, Int 2, Wis 15, Cha 9.
Skills: Hide +0 (+10 in water), Listen +7, Spot +7.
Feats: Improved Initiative, Power Attack.
Advancement: 10–18 HD (Gargantuan); 19–40 HD (Colossal).
Advanced Sea Serpent: CR 10; Colossal magical beast; HD 19d10+133; hp 237; Mas 25; Init +7; Spd swim 60 ft.; Defense 18, touch 5, flat-footed 15 (–8 size, +3 Dex, +13 natural); BAB +19; Grap +51; Atk +27 melee (4d6+16, tail slap); Full Atk +27 melee (4d6+16, bite or tail slap); FS 30 ft. by 30 ft. (coiled); Reach 15 ft.; SQ darkvision 60 ft., hazy outline, improved grab, low-light vision, swallow whole; AL none; SV Fort +18, Ref +14, Will +8; AP 0; Rep +0; Str 42, Dex 16, Con 25, Int 2, Wis 15, Cha 9.
Skills: Hide –8 (+2 in water), Listen +7, Spot +7.
Feats: Cleave, Great Cleave, Improved Initiative, Power Attack.