As with any purchase, the more research you do, the more likely you are to make the right decision. Read some of the canal magazines, visit some of the many exhibitions which are held around the country (see the events calendar) and do go out on the sort of boat you're thinking of, even if only for a weekend. Here are some of the things you should bear in mind:
Different Waterways - The sizes of the canals and river navigation vary, so it is important to buy a craft suitable for all the waterways on which you wish to use it.
How often will you use the boat? - How often you intend to use your boat will affect the type of boat you will need.
Different types of boat - Cruisers and different types of narrow boat.
Sensible maximum length - Locks on the canal network vary in size and where you intend to cruise will affect the length of boat you need.
Fixtures & fittings - Decide on the extent of the fixtures and fittings you need.
How to find your dream boat - magazines, internet, word of mouth, marinas, boatyards and brokers.
Ownership & re-sale - Unlike motor vehicles, boats don't come with registration documents so you need to obtain proof of ownership.
Legal requirements - A cruising licence, Boat Safety Certificate, Valid Mooring (unless you are continuously cruising), Insurance
Buying a boat checklist - Licence, Boat Safety Certificate, Recreational Craft Directive, Insurance, Surveyor