Bad hair day?

Today I got shouted at on the lake! How very rude. I really don't know what I did wrong. I thought it was ok to drive on the left side, depending on convenience. After all, Zürichsee is not very wide, at least not close to the city.
I came driving at a leisurely pace, mainly because my engine is not very big, meeting one of the smaller cruise ships. I saw it, I held to the left and the captain or one of the guys came out shouting something at me when we passed. I did what I usually do when I don't understand things in German, I nodded and smiled. But I still do not know what I did wrong. Was it too close? It's not exactly like they were gonna be affected by the waves I create, rather the opposite actually.

I'm gonna go with that the captain had a bad hair day.