Using a convoy to mask your sound was a historical tactic used by many including Otto Kretchmer. The sound of all that water churning made quite a ruckus to hydropone operators. and it was quite difficult to spot a subs noise in all that background noise.
In game I am doubtful if this works. It did in Aces of the Deep.
Also yes Sonar works worst near the surface and in choppy sea's they have a damned hard time finding you even feet away at periscope depth. near the surface water is always compacting and expanding as well as moving vertically (Waves) not the best way to send sound thru that mess and get a signal back. Also waves have highs and lows and once the sound hits a low it deflects down and gets broken up.
In game this works in stormy weather.
