No, electric eels may kill people making "leaping attacks" but they are in no way the most dangerous sea creatures as they are not responsible for many shallow water depths that occur when people are at the beach swimming or surfing. These attacks, although uncommon, are usually shark related. Sea creatures in general do not cause a lot of harm to humans, but there is no doubt that a shark, much more capable of killing humans or any other animal, is the most dangerous.
The ability to perform 'leaping attacks' does not make electric eels the most dangerous creature in the sea. According to National Geographic, the largest electric eels are capable of emitting bursts of electricity up to 650 volts - five times the power of a standard U.S. wall socket. While direct and sustained shocks of 600+ volts are strong enough to cause respiratory or heart failure in humans, very few deaths via electric eel have ever been reported and the event is considered extremely rare. These creatures are very shy and known to reside in murky, shallow waters, only discharging electricity when actively pursuing prey.
While electric eels are extremely dangerous by making 'leaping attacks' it does not make them the most dangerous sea creature. Shark are the highest sea creature on the food chain and are far more dangerous than electric eels. You do not her of fatal electric eel attacks nearly as much as fatal shark attacks.
Science magazines are literally abuzz with the news that electric eels "leap" to attack their prey. While this is shocking news to many, it doesn't make electric eels any more dangerous. There are animals in the sea both larger and more poisonous than the electric eel, so the eels can't be labelled the most dangerous sea creature by any stretch of the imagination.