U Boat or S E Boat insignia pin...or...what is it?

[grimble-nibitz]

I posted this in the German area, perhaps by mistake, sorry! Does anyone know what this piece is? Its the size of a US half dollar plus a bit, its heavier than aluminum, and seems to be Kriegsmarine, nautical in nature...perhaps a logo fromn S-Boat unit? No info on this seems to be online...this came from a WWII POW veterans lot here in the US, along with other nazi era material and his POW camp material (Stalag VII/A) Thank you in advance for any help or direction you may be able to give...because I am lost on this one. THX!