If you're serious about living as close to the earth as possible, living on a boat is a good step. Our ecological impact is lower than that of a house, although we tend to depend pretty heavily on fossil fuels: coal, gas, diesel. A few conscious choices can reduce the ecological impact of living on board and save you quite a bit of money in the process.
For everything you could want to know about ecological boating, visit the excellent Low Impact Life Onboard. Meanwhile, here are a few simple steps you can take, in a very rough suggested order of priority.
Heating in winter is important not just for you but for your boat, too; it keeps back the damp.
If you can, use wood, charcoal or 'golden coal' (highly compressed sawdust) from a sustainable source, but smoke from all these makes them less suitable for built-up areas.
If using smokeless coal, go for a high-quality type. Homefire, for example, burns cleaner, hotter, and for longer than some of the other smokeless coals, and so is worth the extra couple of quid per bag.
The carbon emissions of a winter's worth of smokeless coal for a live-aboard boat are roughly equivalent to one passenger's share of a return flight from London to New York. You can reduce this to a return trip to, um, Ascension Island by using wood most of the time and keeping smokeless coal for the coldest nights.
When loading your fire, add small amounts of fuel at a time. Make sure your fire burns hot, as otherwise the fuel won't burn cleanly, but don't blast the berjeezers out of it - never let it get red hot. See How to make a really good fire.
An alternative is diesel-based heating using a sustainable form of biofuel (although many types of biofuel are not sustainable). The best kind of biodiesel is reclaimed oil (e.g. reprocessed chip fat), if you can get it.
Compare the CO emissions of fuel types for heating on the StovesOnline website, which shows that wood is by far the best of the lot.
Gas and electric fridges use a lot of energy, even the more efficient ones.
For much of the year, the bottom of a low cupboard is cool enough to use as a larder for things you'd otherwise keep in a fridge. If you have access to the baseplate (the bottom of your boat's hull) then food containers stored there will stay cool at any time of the year. In the summer, use the canal to keep beers etc. cool. It's just as good as a fridge on a hot day - well, nearly.
If you can get it, try a sustainable form of biodiesel (see above).
When travelling, don't thrash the engine - it uses a lot of fuel and most of the energy is lost making a big bow wave.
The best thing you can do is to use less of it. Defenestrate the telly, decommission your curling tongs. It's also well worth switching to good LED lighting throughout, at a cost of about £20 per light - they use barely any energy at all.
The next best thing is to keep your batteries in good nick. Once batteries start to lose their joie de vivre, they don't hold their charge as well and you spend a lot of fuel trying to keep them going. If you treat 'em mean, they'll soon be lasting you just half an hour at a time. So, never let your battery go below 50% and keep it nearer to 80% if you can.
Solar panels have finally become cost-effective for boating and thither waits the future. An outlay of about £250 plus fitting costs will keep your batteries topped up most of the time for most of the year. The panels will pay for themselves by keeping your very expensive batteries going for a year or two longer. Wind turbines are an option too but they don't seem to be as effective and they can get in the way (and sometimes even fly apart).
If you run mains appliances like a laptop from an inverter then switch if off when you're done - it uses power even when the appliances are turned off.
When running your engine to charge the batteries, charge everything else at the same time - laptop, phone, whatever. The engine produces excess energy and you might as well make use of it.
Finally, when charging the batteries, run the engine at just enough revs for the charging indicator to reach its highest point, then slow the revs down as the battery charges up, but not too slow because that's bad for the engine.
If you don't have a separate oil tray to catch drips from the engine, you won't be given a Boat Safety Certificate (unless your examiner is a charlatan, in which case you might). Pump any spilt or old oil into a container and lobby the Canal and River Trust to offer places to leave it for collection, because they don't really offer that at the moment.
As for the rest of the bilge, it doesn't have to be as clean as Lourdes water to be pumped out into the canal, but if there's a film of diesel or oil on the surface, you'll need to pump that into a container as well.
If you ever see a swan with a red tinge in its wings, it's because it has been poisoned by oil/diesel on the canal.
You're allowed to tip household waste water (so-called grey water) into the canal. To reduce the impact use ecologically friendlier detergents, like Ecover for the dishes and pure soap for yourself (because you're worth it). If you love your hair, consider using a leave-in conditioner rather than a wash-out one.
For the loo, chemical with low or no formaldehyde is available from any good chandlery (and not from any bad one). Tipping shit overboard is illegal, a serious hazard to wildlife and obviously disgusting, but you know that.
Visit Low Impact Life Onboard.