Sonning Lock

Anyone living on an upper floor of the building in the background could have seen this from his or her balcony.

No problem with this. I was referring to someone standing directly outside the window of a moored boat. Your actions might be totally innocent (and legal), but could be disconcerting for the person inside. However, he should have accepted your explanation wihout being abusive.

Houseboat dwellers, like some landowners, can easily be as territorial as rabbits.

And rightly so. It's their home.