Yep, there's no demo available.
But idea to watch some videos made by dedicated kaleuns is great!
Best download area is Terrapin's site:
http://u-boot.realsimulation.com/
Click "SH3Downloads", agree with terms of use and then select "Silent Hunter 3 Video Downloads".
IMHO "Sink the tanker" and "Blind Man's Tales" are exceptionally good ones.