As long as it's decaying "hot" enough to boil something, then you can get energy out of it. And using static reflectors and good insulation, you could pretty much use glow-in-the-dark wristwatches for energy.
Here's a thought: use some energy to keep the fuel molten and in a tray over the water supply. The water takes some heat from the bottom of the tray and uses that for power. While that's happening, the fission products, being lighter, float to the top of the tray and are skimmed off, while the remaining useful fuel is denser (having not split) and remains at the bottom of the tray.
Also, when people talk about hydrogen for fuel, they're talking about fuel cells, not fusion. Fusion is almost certainly not possible yet. Superconductors may make it possible.
The best power source I know of is one we can already do: A huge solar power station at each of the Earth-sun lagrange points (well, the stable ones, at least), beaming power by microwave. Solar in space has none of the restrictions that earth-bound solar has, never gets night, never gets snowed on...
They're already planning merely geostationary solar at this very minute. It has 75 minutes of night per year. If we set up 6999 of these satelights, we'd have percisely one in the shadow at all times, so there'd be no fluxuation, just constant solar power, and a hour or so for down time per year.
And don't tell me you played Sim City and your cities always got microwaved when you built those, because that is the first thing anyone says, so can't you think of any real reason that this wouldn't work?