Sure, great idea! I'll boost Cha to 18 to keep AC constant. It's not like they aren't charismatic.
Updated.
Shall we give them elf traits (low-light vision, bonus vs. enchantments, etc.)?
Combat: In their human form, ice vampires can manipulate cold, which gives them the following powers, each usable once per day: ice storm, cone of cold, and waIl of ice, as 7th-level spellcasters.
This seems pretty straightforward...
Spell-Like Abilities: 1/day—cone of cold (DC x), ice storm, wall of ice. Caster level 7th. The save DCs are Charisma-based.
Their touch drains 2d4 hit points of warmth, which are added to their hit point total, to a maximum of 50 points above their normal hit point maximum. These extra hit points fade away after 24 hours.
So, literally cold damage that heals the ice vampire, or something more traditional like Con damage/drain (with the usual temporary hit points)?
In their pool form, if they position themselves under a waterfall, they can enthrall one creature within a 240-foot radius; this includes creatures that are normally immune to charm (such as elves). The creature targeted must roll a saving throw vs. spell; if the roll fails, the creature is drawn to touch the water, losing 2d4 hit points per round until he dies or is pulled away by others (these hit points vampirically restore the hit points of the ice vampire, as above). It takes a round for the ice vampire to change forms.
Alternate form?
Something like this?
Beguiling Song (Su): A rusalka can attempt to beguile creatures with its song. The rusalka sings, targeting a single creature it can see within 300 feet of her body of water (see water symbiosis, below). This is a sonic, mind-affecting ability, and the creature must be able to hear the rusalka for it to take effect. The targeted creature must make a DC 17 Will saving throw. If the save is successful, that creature cannot be affected by that rusalka's song for one day.
Failure indicates the creature is utterly beguiled and moves toward the rusalka, taking the most direct route available. If that path leads into a dangerous area (such as very deep or fast-moving water) the beguiled creature gets a second saving throw. A rusalka who desires a companion often moves out of the water and bestows water breathing upon its beguiled victim so that she can take him underwater. A beguiled creature may take no actions other than defending himself. A beguiled victim who moves within 5 feet of the rusalka must make a DC 18 Will saving throw or be charmed as per charm monster cast by an 8th-level sorcerer. Success means that the character is freed from the beguilement and is immune to that rusalka's beguiling song for 24 hours. The beguiling effect continues so long as the rusalka sings. A rusalka does not need to continue to sing to keep a victim charmed.
A rusalka can use her beguiling song both above and below the water. Good rusalkas cast water breathing on their charmed companions to keep them alive below the waves. Evil rusalkas move into deep water, forcing a beguiled victim to move toward them but never allowing the victim to get within 5 feet. If these unlucky creatures fail their second saving throw, they usually drown (see page 304 of the DMG).