Wikipedia has a "List of Missing Aircraft". Admittedly, most do not fit the criteria of "airliners that disappeared without a trace at sea", but some do. (BSAA Star Ariel is a famous one) The Varig 707 case is instructive because it happened so close to Tokyo. If a large jet could disappear without a trace there, imagine how much easier it would be in a thousand mile swath thru the middle of nowhere...