I would suggest you or anyone in a similar position, join a local
Kayak canoe group

http://www.canoe-england.org.uk/cano... nuItemId=597

I joined one for about £50 a year, and then borrowed their
equiptment, it is much safer than paddling on your own with your son

What is the cheapest possible usable boat that you can buy? At least at
first, I wouldn't be interested in serious boating. But if I had a
small thing that I could take to the local canal and row up and down a
bit it might be fun and good exercise. I have a young son and I think
he would enjoy it. I notice that Millets sells a small 1 1/2 person
inflatable boat.

http://www.millets.co.uk/millets.sto...G+Base+Catalog)

Would such a thing actually be usable? A local pound-shop-like place
has a small inflatable for £5 but even in my ignorance I'm confident
that would be useless for anything.

Also, is it legal to just go to the local canal (Grand Union Canal) and
just start rowing in it? Do you need a licence?