1WO means...

[JScones]

Originally Posted by Markus Witt

Anybody knows how many watch officers U-boats normally had and what watch system they used?

4 Watch Officers, with 3 and 4 being split between Nav and Bosun. 4 hour watches.

1WO and 2WO did 2 x 4 hour watch shifts per day <---Commissioned officers from line stream
Navigator (Oberstuermann) and Bosun 1 x 4 hour watch shift per day <---Yes, SNCOs, not commissioned officers, aka 3WO and 4WO
Lookouts came mainly from Seaman stream doing 1 x 4 hour watch shift per day

As confirmed in the first two reference books I just laid my hands on:
U-boats by Miller, David
U-boat Crewman Of WWII by Williamson, Gordon