What fish can eat a human in seconds?

Shark,

Piranha(although they attack only in packs),

Some very big Catfish (http://www.snopes.com/photos/animals/giantcatfish.asp) ,

Some huge Squids (http://www.msnbc.msn.com/id/17275072/ns/world_news-world_environment/t/huge-squid-caught-could-be-biggest-ever/#.UPuyFazCa04)

We really don't know that much about ocean,some scientist say that we know more about the Universe than we know about the depths of our seas.

You could watch Jeremy Wade's tv series :"River monsters" to find more man-eating fish.