Hello Cinzia! The expression “below the waterline” is connected to the iceberg metaphor (please see the second link attached in english and the “Short explanation of "The Culture Iceberg" theory from Anthropologist Edward T. Hall) and it refers to the hidden part of the iceberg below the surface. So it refers to unseen, internal, deep, invisible, hidden cultural aspects or parts of a matter/study.... Those are the most difficult to observe. I would translate it as above: I included different ways you can word it but the idea is “go deeper”.. below the waterline. I hope this can help! Naomi H.