The other half of the MM's nagas.
Pictured above, these 9 HD monsters are similar to the dark version in a lot of ways. Their stats are probably slightly worse, as are their natural attacks, but they have a continuous charming gaze (not friendly, but that doesn't really matter) and the ability to add cleric (and Chaos domain, and Evil domain) magicks to their spells known.
That said, giving +1 LA would make spirit nagas incapable of getting 9th-level spells, which as we've established before isn't really something any of the nagas deserve. +0 LA.
The only Neutral ones of the bunch. Water nagas are slower on land than the others, but can swim reasonably fast. They also have the best sorcerer casting in terms of HD, with CL 7 and as many hit dice. Their charisma is low compared to other nagas, though.
The sorcerer casting equal to HD, combined with the reasonable stat boosts and poisonous bite, is enough for me to grant water nagas +1 LA. Do note that they can't breathe air without magical assistance, though.
Night hags are next.