D&D also has Nagas, including Water Nagas.
http://www.d20srd.org/srd/monsters/naga.htm#waterNaga
However, they don't even have an LA, which means they were never intended to be played. Any DM allowing it would have to make adjustments. (Bit too strong for it to really have +0 LA.)
Lamia Nobles (NOT regular Lamias) are also snake-like, but I seem to have displaced my books and they're not in the SRD. Pretty sure those are also evil though.
In Savage Species there is a template to turn any animal anthropomorphic, you could play an actual snake turned human. Do note that Savage Species is sort of "3.25", stuck between the two editions, so may require a little fixing.
Serpent Kingdoms apparently has (I read so, don't remember) a Replacement Tail graft, which replaces your legs with a huge snake tail. Kind of a poor man's version. (Not a literal poor man though, it's costly.)
Also I hear PF Merfolk have a "strongtail" version, which is slightly less fish-like and more snake-like. Still lame though.
That said, although most of these are "evil", this is D&D. There can certainly be exceptions. *cough* Drizzt *cough* You want to convince a DM to let you play one of these, try to find one who reads the webcomic Goblins.
SPOILER:
There's a really nice female Yuan-Ti. Smart too.