I found out, that there was one thing missing in the Map: To show one's actual position! Again I googled around. Soon I found the neccesary Maps API and started implementing. After some tries it worked. The Map now automatically sets a red Google-Marker at your actual position if your device supports GPS. When you move around, the Map moves with you. So, basically, the Map now acts just like a Chartplotter. Cool!
I tried wit several mobile devices (went to the next shop, tried in all devices they had), and also tried the Location Awareness at a car ride on the highway from Graz to Vienna. It also worked at 80 mph, more than enough for sailors.