If you are on a permanent mooring, then you get a water hookup or you go fill up water tanks as you empty the waste? If you are moving around then how often do tanks need filling/emptying?
Some moorings come with water, some do not. We used to fill out water tanks every two weeks. Your potential water tanks might be larger/smaller. You might use less water than a family of three, you might use more. Difficult to answer with any definates.
Are some waterways/canals narrower than others? Presumably so. Is there a site that can tell me where a boat of a particular length/width can go?
Yes some canals have narrow locks (about 7-8ft wide) which will limit your cruising if you're a wide beam. Also some canals have a max length due to limitations in the locks. A bit of googling should tell you what's what. I think Wikipedia might list the max lengths for the canal if you look up the specific page, e.g. https://en.wikipedia.org/wiki/Kennet_and_Avon_Canal
Do you need the full amount up front to buy a boat? ie you cant get a mortgage on a boat, right? Who pays for an out of water survey? The buyer or seller?
Marine mortgages exist - that's the limit of my knowledge about them though. The buyer pays for the survey same as when buying a house. You want the surveyor to be working for you, not the vendor!
Security. I see a lot of boats just on the side of the river/canal as opposed to in a secured marina/mooring. They look like they are unattended for days if not longer. Isnt there a risk of break-ins?
As with anything, there is a risk of crime. Obvious things like don't leave your generator on the deck with easy access make you less of a target, lock up your bikes properly, don't leave a laptop on show in the window... I think crime rates do tend to be lower on the canals but you hear about the odd incident.