I lived on a boat in South Dock Marina for three years, and it's not bad. Good facilites, 10 minute walk to the Jubilee Line and a water bus stop right outside.
You'll be hard pushed to find any long term mooring in Camden. Even if you do you can be certain that it won't be cheap!
The British Waterways web site www.waterscape.com
might be worth a try. If you're really serious about it, then go and visit anywhere that you think might be likely and talk to people. It's not unusual for vacant moorings to go by word of mouth without being advertised.