Well, having lived on a narrowboat for the last 6 years, I would say go for it. A little cabin cruiser need not be expensive, perhaps something like a Dawncraft, somewhere between 16 - 20 foot. If you have a driveway you can tow such a small boat out of the water with a car, thereby saving a fortune on moorings. You could get a serviceable cruiser in this league for less than £2000.
Although saying that, I have as much fun on my canoe, which can go to most of the places a powered boat on the Thames can and lots they can't and cost £400 secondhand, with no petrol or maintenance costs. Insurance and a gold license (the equivalent of road tax) is £35, whereas a powered boat will be at least five times that.
I'm just outside Oxford on the Thames tonight, about to go up Duke's Cut in the morning. If you are near and want a chat about boats, drop me a line.