vonBraunau
As AvonLady already mentioned most of questions are already answered in the Community Manual. I'd recommend searching this forum, but the search engine appears to be broken :|
As for the Flak guns and waterborne targets - with the 2cm light flaks you only can sink smalest ships - Elco torpedo boat, small coastal veesel, tug boat, trawler (not armed trawler

) and a fishing boat. They can be usually found close to the shore and in harbours - don't expect to see a fishing boat in the middle of the ocean

With the 3,7cm heavy flak and it's AP ammo you can sink any ship, but be prepared that it can take a lot of time and a lots and lots of ammo (100+ rounds for a small merchant). So I don't recommend trying to sink a King George V class BB with it