The update added advanced visual details including:
Realistic underwater particles enhance the sub-surface atmosphere.
The periscope eyepiece now features raytraced reflections from the lens for increased fidelity.
Heavily armed German warships now occasionally accompany you, particularly during Mediterranean missions.
For more details, you can view the official B131 Update Trailer or read the full patch notes on SteamDB . UBOAT - Christmas update: B131 - SteamDB