Sea snakes in real life are relatively common only in certain waters. While it is true they are venomous, and some extremely so, deaths from encountering these reptiles are fairly rare.
In classic D&D, sea snakes are an unusual encounter even in an aquatic environment. Their low morale means they aren't going to hang around a fight for long, either. There are a couple things that make this little fella insanely scary, though.
Like all poisonous snakes, you've got a save or die potential. However, with sea snakes, there are a couple of oh-so-fun twists to keep in mind.
- Half the time, the victim doesn't even necessarily know he's been bitten (tiny teeth).
- Even if Neutralize Poison is used, 25% of the time the spell fails vs. the venom's potency. That's right. A one in four chance that 4th level clerical spell designed to stop poison from killing you just won't cut it against this stuff!
In the game, sea snakes are aggressive and may even attack a human for food (Moldvay). While I don't know that this makes real-life sense (snakes swallow their prey whole, and a human is a tad big for that), it makes having 1d8 of them swimming toward you more than a bit unnerving.