Don't mix your sugar and egg yolks too long before your temper them with cream, or there's a chance they could turn green.
1 more tip: after you add your ramekins to your water bath, you may notice some tiny air bubbles on the surface of your creme brulee. Take your blowtorch lightly over the tops of your brulees, just long enough to burst the tiny bubbles on the surface; you don't want to cook the tops of them. The reason for doing this is because the air bubbles will trap heat near the surface and you'll end up with a weird crust on the tops of your brulees. It isn't very tasty and it definitely impedes your ability to put a proper sugar crust on it after they are done baking.
1 more thing now that I think of it- and this varies dependingon your recipe- if your water bath pan is deeper than your brulees are tall, consider placing a sheet pan over 80% or so of the top of your water bath pan. Sometimes creme brulees will souffle' a little bit, and this helps to prevent that, though I'm not sure why. I think this is more of a risk if your recipe contains egg whites, however.
I also use cold water for my water bath, to ensure even heating. I don't imagine it would be a problem to start with boiling water, so long as you adjust your cooking time. If they bake for too long you'll end up with vanilla scrambled eggs.