Merrow (CR 3)

Large Giant (Aquatic)
Alignment: Usually chaotic evil
Initiative: -1 (Dex); Senses: darkvision 60 ft., low-light vision, Listen +2, and Spot +2

AC: 16 (-1 size, -1 Dex, +5 natural), touch 8, flat-footed 16
Hit Dice: 4d8+11 (29 hp)
Fort +6, Ref +0, Will +1
Speed: 40 ft. swim
Space: 10 ft./10 ft.
Base Attack +3; Grapple +12
Attack: Longspear +8 melee
Full Attack: Longspear +8 melee
Damage: Longspear 1d8+7
Special Attacks/Actions: -
Abilities: Str 21, Dex 8, Con 15, Int 6, Wis 10, Cha 7
Special Qualities:
Feats: Toughness; Weapon Focus (longspear)
Skills: Listen +2, Spot +2, and Swim +5
Advancement: By character class
Climate/Terrain: Temperate aquatic
Organization: Solitary, pair, gang (2-4); or band (5-8)
Treasure/Possessions: Standard

Source: Monster Manual

Merrow favor overwhelming odds, sneak attacks, and ambushes to a fair fight. They are intelligent enough to fire ranged weapons first to soften up their foes before closing, but ogre gangs and bands fight as unorganized individuals.

Aquatic Subtype

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.