Monster manual, under elf, pg 84. They're usually impraticle because they can't survive out of water for more than [Con Score] hours.
Edit: But hopefully Faraear won't have that problem! Here he is, in all his aqueous glory.
Faraear: Male Sea Elf Ftr5; Medium Humanoid; HD 5d10+15; hp 47; Init +4; Spd 30, Swim 40; AC 19 (+4 Dex, +5 Chain shirt +1) ; Atk +10 Melee (1d8+7, +1 Trident), or +10 ranged touch (Masterwork Net) SQ Sea Elf Traits; AL CN; SV Fort +7, Ref +5, Will +2; Str 18, Dex 18, Con 16, Int 6, Wis 11, Cha 8.
Skills and Feats: Climb +11, Spot +2; Ambidexterity, Exotic Weapon Proficiency: Net, Focus: Trident, Specialization: Trident, Point Blank Shot
Possessions: +1 Trident, +1 Chain shirt, Quaal’s token (Anchor x3), Potions of Cure Moderate Wounds (x2), 5 Javelins, 20 gp gem, 2 gp, Masterwork Net
Sea elf traits: As per elf, plus:
Swim 40 feet
Low-light vision 4 times as far as human
Breathe underwater, survive as long as 1 hour per point of consitution above water.