All canal boats have chemical toilets of one sort or another. Sea toilets, which discharge overboard, are not allowed on inland waterways.
There are two basic types:
Recirculation toilets - hold the treated waste either in a large sealed tank on board or in a smaller tank incorporated into the toilet unit. If you've got a sealed holding tank on board you'll need to find a pump-out point to empty it. These can be found at most boatyards, but there is usually a charge for pumping out; hire companies may have reciprocal arrangements with others in their area, or may refund any charges.
'Bucket and chuck it' - Less popular, though eminently practical and simple to use are the 'bucket and chuck it' variety - there's no pump to fail and they're very easy to clean. If you've got a portable unit, you should look out for the small brick or concrete buildings labelled 'sanitary stations'. Some of these are locked, so you need to make sure that you have a key. Your hire company will supply one, but if you're cruising in your own boat, the information that come with your licence will tell you where to get a key.
Disposal points are indicated in navigation guides and will usually have water taps and rubbish bins at the same site. Organic toilets are also available.