Running at 'decks awash' is generally used by people in their gameplay so they can dive quicker when aircraft are spotted. An obvious adittional benefit must be a lower sillouette for ships to spot. When your torpedo impacts on the merchants the initial giveaway to the escorts should probably be from which side of the convoy column you are firing from. This will probably assist them in searching an area they estimate you to be in.
My moral is: SHOOT THEN DIVE !!
