Your experience of float flying schools differs significantly from mine. At all three I have attended, step taxiing has been taught - it is after all an essential skill.
The post rating training from all included curving and circular take-off and landing. The commercial rating from two of the schools had a significant amount of 'confined area' operations as well.
With the foregoing, you obviously received lots of good training, and that is excellent. These would all be trained, but at the rather advanced stage of float training. Certainly, in Canada, many float pilots would be flying without ever having received this training, it is not mandatory.
Certainly step taxiing is a valuable skill, and I do it often, but in a floatplane, with caution. I'll ask at the local float flying school what they do train for this, but I think I recall that solo rental step taxiing was prohibited in years past.
Having looked at the Thames on Google earth to remind myself (its bee a few years since I was there), there are a few places I could get in and out happily space wise, but I still would feel uneasy traffic, and built up area wise. There are some things which are possible, which might not be worth the cumulative risk - it looks to me that landing on the Thames might be one of them. We floatplane pilots do ourselves no favours, letting the public see the planes doing things they see as intrusive or unsafe in a built up area.