Dragon, Black Young (CR 4)

Medium Dragon (Water)
Alignment: Always Chaotic Evil
Initiative: +0; Senses: blindsense 60 ft., darkvision120 ft., low-light vision, and keen senses
Languages: Draconic

AC: 19 (+9 natural), touch 10, flat-footed 19
Hit Dice: 10d12+20 (85 hp)
Fort +9, Ref +7, Will +7
Speed: 60 ft., fly 150 ft., (poor), swim 60 ft.
Space: 5 ft./5 ft.
Base Attack +10; Grapple +12
Attack: 1 Bite +21, 2 claws +16, 2 wings +16 melee; Breath +21 ranged
Damage: 1 bite 1d8+2, 2 claws 1d6+1, 2 wings 1d4+1, Breath weapon 6d4 (17)
Special Attacks/Actions: Breath weapon
Abilities: Str 15, Dex 10, Con 15, Int 10, Wis 11, Cha 10
Special Qualities: Acid immunity, water breathing
Feats: #Feats: 4
Skills: Skill points: 6
Advancement: 11-12 HD (Medium-size)
Climate/Terrain: Any marsh and underground
Organization: solitary or clutch (2-5)
Treasure/Possessions: Double Standard

Source: Monster Manual

Breath Weapon (Su): A black dragon has one type of breath weapon, a line of acid. Using a breath weapon is a standard action. Once a dragon breathes, it can't breathe again until 1d4 rounds later. A blast from a breath weapon always starts at the dragon's mouth and extends in a direction of the dragon's choice. The black dragon's breath weapon deals 6d4 acid damage. Creatures caught in the area can attempt Reflex saves (DC 17) to take half damage.

Water Breathing (Ex): The dragon can breathe underwater indefinitely and can freely use its breath weapon, spells, and other abilities while submerged.

Spell-Like Abilities: 3/day - darkness (radius 30 feet).

Water Subtype

This subtype usually is used for elementals and outsiders with a connection to the Elemental Plane of Water. Creatures with the water subtype always have swim speeds and can move in water without making Swim checks. A water creature can breathe underwater and usually can breathe air as well.