Ocean Strider (CR 18)
AC: 19 (-2 size, +1 Dex, +10 natural), touch 9, flat-footed 18
Hit Dice: 30d6+60 (165 hp); DR: 15/cold iron
Fort +12, Ref +18, Will +23
Speed: 50 ft., swim 60 ft.
Space: 15 ft./15 ft.
Base Attack +15; Grapple +18
Attack: Masterwork falchion +18 melee and tail slap +12 melee, or Masterwork trident +18 melee and tail slap +12 melee, or tail slap +17 melee
Full Attack: Masterwork falchion +18/+13/+8 melee and tail slap +12 melee, or Masterwork trident +18/+13/+8 melee and tail slap +12 melee, or tail slap +17 melee
Damage: Masterwork falchion 2d8+6/18-20, Masterwork trident 4d6+6, tail slap 1d6+2 (secondary) or 1d6+6 (primary)
Special Attacks/Actions: Frightful presence, ramming, spell-like abilities
Abilities: Str 19, Dex 13, Con 15, Int 16, Wis 22, Cha 14
Special Qualities: SR 28, amphibious, water walk
Feats: Alertness; Blind-fight; Cleave; Combat Expertise; Dodge; Improved Initiative; Imporved Improved Sunder; Mobility; Power Attack; Spring Attack; Whirlwind Attack
Skills: Hide +26, Intimidate +35, Jump +37, Knowledge (nature) +38, Listen +41, Search +36, Spot +41, Survival +39, and Swim +45
Advancement: By character class
Climate/Terrain: Any aquatic
Monster Manual II
Frightful Presence (Ex): The mere presence of an ocean strider inspires terror in all creatures within 100 feet of it that have fewer Hit Dice or levels than it has. Each potentially affected opponent must succeed at a Will save (DC become shaken - a condition that lasts until the opponent is out of range. A successful save leaves that opponent immune to that ocean strider's frightful presence for 24 hours.
Ramming (Ex): As a standard action during its turn round, an ocean strider can swim at up to quadruple speed (240 feet) and ram a waterborne target (such as a another creature). To ram, the ocean strider must end its movement in the target's space. This attack deals 2d8+6 points of damage. If the target is a creature, it can attempt either an attack of opportunity or a Reflex save (DC 29) for half damage.
Upon ramming a ship, the ocean strider can make a Strength check to breach its hull, which causes the to sink in 1d10 minutes. The break DC varies with the type of vessel rammed, as follows: rowboat DC 20, keelboat DC 23, sailing ship or longship DC 25, warship or galley DC 30. (See Chapter 5 of the DUNGEON MASTER's Guide for information about ships.) Regardless of the check result, every creature aboard must attempt a Reflex saving throw (DC 15). Success means the creature takes 1d10 points of damage from being thrown about by the impact; failure means the creature is hurled overboard.
Spell-Like Abilities: 6/day - control water, fog cloud, storm, obscuring mist; 5/day - acid fog, cone of cold, elemental swarm (water only), horrid wilting, summon nature's ally IX (water or aquatic creatures only). Caster level save DC 17 + spell level.
Water Breathing (Ex): An ocean strider has gills as well as lungs, so it can breathe water as well as it can air.
Water Walk (Su): An ocean strider continuously produces an effect like that of a water walk spell (caster level 20th).
In battle, an ocean strider first uses its powerful spell-like abilities to confuse, disorient, and soften up opponents. Then it uses the Spring Attack feat to enter combat with its weapon.
Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.