So deep water can be chosen in C M F J D and secure flanks . Its placed on one of the flanks and a roll of 3-4 it's lost as no impassable terrain? Now what had me wondering is : "may be used as a secure flank"
What does that mean exactly that when you have a river or coast secured flank you may use a deep water terrain or that if you manage to put a deep water terrain on a flank it becomes a secure flank even if it was not
Deep Water secure flank is an outcome of the Pre-Battle Terrain System. It is placed automatically by the defending player, there is no roll involved. It extends the entire length of the short table edge.
A deep water terrain selection (essentially a pond or lake) is a normal optional terrain piece selection. You roll as normal, but cannot be placed in the central sector (a roll of 3 or 4) in which case it would be lost. On a roll of 1,2,5 or 6 it is placed (if space allows) by the choosing player and the opponent has the usual roll to shift (4,5,6), pivot (5,6) or remove (on a 6).
Nothing else is implied or intended.