I am not sure how to update from 3.5 to Pathfinder. I didn't have a Rhine Maiden that I could find, so I decided to throw one together:
Medium Fey (Aquatic, Shapechanger)
Hit Dice: 9d6+9 (40 hp)
Speed: 30 ft., Swim 60 ft.
Armor Class: 15 (+2 Dex, +3 Deflection), touch 15, flat-footed 13
Base Attack/Grapple: +4/+6
Attack: Rapier +6 melee (1d8+2/18-20)
Full Attack: Rapier +6 melee (1d8+2/18-20)
Space/Reach: 5 ft./5 ft.
Special Attacks: Death Throes
Special Qualities: Alternate Form, Amphibious, Break Enchantment, Guardian of the Rhine Gold, Low-light Vision, Immunity to Cold, Unearthly Grace, Transparent, Vulnerable to Desiccation Damage, Water Dependent
Saves: Fort +7, Ref +11, Will +8
Abilities: Str 14, Dex 15, Con 12, Int 13, Wis 8, Cha 16
Skills: Diplomacy +5, Gather Information +15, Knowledge (Local) +13, Listen +11, Move Silently +14, Sense Motive +11 , Spot +11, Swim +14
Feats: Ability Focus (Wrath of the Rhine Maiden), Combat Reflexes, Dodge, Weapon Finesse
Environment: Temperate Rivers
Challenge Rating: 6
Treasure: Standard (50% uncut gems and gold nuggets)
Alignment: Usually Neutral
Advancement: By character class
Level Adjustment: —
In their natural form, Rhine maidens are indistinguishable from the water they inhabit; only the faint outline of a titillating female figure gives away their presence. When they choose, they look like pale, water-drenched human women with long flowing hair. Rhine maidens are graceful, beautiful, and passionate. They are also quick to anger. They prefer short, intense dalliances with a variety of suitors, often instigating jealousy between them.
In sharp contrast to their wanton behavior, Rhine maidens take stewardship of their Rhine gold very seriously, abandoning even the most ardent partner at a moment's notice if need be.
The rivers inhabited by Rhine maidens are noted for their gold deposits. Anyone wishing to pan for gold in a Rhine maiden's river must first receive her permission or risk her wrath.
Rhine maidens stand 5 feet tall and weigh 120 lbs. They speak Common and Sylvan.
Rhine maidens are loathe to fight unless their Rhine gold is threatened, in which case they do not hesitate to battle to the death.
Alternate Form (Su): A Rhine maiden can shift between its humanoid and water forms as a standard action without provoking an attack of opportunity. Only a specific female human form can be assumed.
Amphibious: Rhine maidens breathe both air and water.
Break Enchantment (Sp): Once a day, as a standard action, a Rhine maiden may cast break enchantment as an 18th level sorcerer.
Death Throes (Ex): When slain, a Rhine maiden bursts into a geyser of water causing 1d4 points of damage to anyone within 20-foot radius.
Guardian of the Rhine Gold (Su): Rhine maidens are tasked with guarding a magical lump of gold that that can be turned into a magic ring but only by someone who first renounces love. Anyone who steals the Rhine gold becomes cursed, taking a -4 penalty on attack rolls, saves, ability checks, and skill checks until the gold is returned or until the curse is broken. The curse cannot be dispelled, but it can be removed with a break enchantment, limited wish, miracle, remove curse, or wish spell. Removing the curse renders any ring made from the Rhine gold non-magical unless a Rhine maiden breaks the enchantment.
Transparent (Ex): While in the water, a Rhine maiden is difficult to see, and it takes a DC 15 Spot check to notice one.
Unearthly Grace (Su): A Rhine maiden adds her Charisma modifier as a bonus on all her saving throws and as deflection bonus to her Armor Class. (The statistics block already reflects these bonuses).
Vulnerable to Desiccation Damage: Rhine maidens take 50% more damage from any spell, spell-like or supernatural ability that causes desiccation damage.
Water Dependent (Su): A Rhine maiden is mystically bound to a single river and must never stray more than 300 yards from it. Any who do become ill and die within 4d6 hours. A Rhine maiden's river does not radiate magic.
Wrath of the Rhine Maiden (Su): At will as a standard action, a Rhine maiden may focus her anger against any foe she can see, gaining a +1 morale bonus to attack rolls and weapon damage rolls against that foe. These effects last for as long as the foe remains in line of sight of the Rhine maiden or until she chooses, whichever comes first. In addition, that foe must succeed on DC 19 Will save or be shaken for 1 hour. A successful save negates. The save DC is Charisma-based and includes a +2 feat bonus.
Edits: Added Transparent and made it clear that wrath lasts as long as she has line of sight of her foe or until she chooses, whichever comes first.
In Pathfinder it seems she'd be CR 5.