I got a boat the other day from London to Greenwich, I was with friends and it seemed like a great idea. It was.
The boat was a City Cruises one and we sat in the open upstairs deck. There was a commentary from someone as we were going along and while light it was pretty informative and entertaining.
The only downside was the cost, £7 or so. However I think that for not much more you can get a return or an all day ticket so if you're having a whole day out it's not too bad at all.
It's definitely a lovely way to view the city and it was much nicer than getting a train.
Anyhow as we approached Greenwich I took a snap of the old Naval College, I've walked past it many times but it was great to see if from a different perspective.