|MadSci Network: Zoology|
It's hard to answer your question with certainty, because there are thousands of marine animals that we've only seen, never examined, and thousands more that have only been studied once or twice. Of the ones we know about, the most venomous are the sea snakes of the Laticauda genus.
This is a picture of a banded sea snake, Laticauda columbina or columbrina, from the Monterey Bay Aquarium.
More notes on their exhibit of venomous sea creatures, which might be of interest to you, can be found at http://www.usw.nps.navy.mil/~millercw/aq/DB.html.
Sea snakes generally eat fish, paralyzing the fish with their venom, which contains a strongly concentrated neurotoxin. For comparison, most terrestrial venomous snakes have poisons for which the LD50 (lethal dose for 50% of test subjects) is from 4000 g to 6000 g per kg of body weight of the bitten animal. In Laticauda, the LD50 ranges from 60 g to 600 g per kg of body weight of the bitten animal. Note that this can be a little misleading, as some snakes inject more venom than others do, and therefore snakes with weaker toxins can actually be more deadly--but sea snakes are still the most venomous things in the sea.
To answer your last question, yes, humans have died from Laticauda bites. There is an antivenin, but not everyone can get to it in time, and there are many areas where it isn't available. Reportedly, though, sea snakes (like most animals) will only bite when roughly handled or attacked.
A box jellyfish, from Australia or Asia, is the deadliest sea creature, and in fact, the world's most venomous "animal".
Try the links in the MadSci Library for more information on Zoology.