How do waterproof fuses work (like, say, on seal charges)

I've heard of charges meant to be used underwater, and that the fuse can't be put out with water.

How does that work?

I used to own waterproof fuse. In a day when you could walk into your local surplus store or gun and knife show and find the stuff. Before if you owned it it must have been for making bombs to blow things up.

The fuse was cable-like and tough. It was a magnesium core filament surrounded by braided plastic. It burned at a consistent rate, and would burn with very little oxygen.

http://en.wikipedia.org/wiki/Magnesium
"Magnesium is a highly flammable metal, but while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk. Once ignited, it is difficult to extinguish, being able to burn in nitrogen (forming magnesium nitride), carbon dioxide (forming magnesium oxide and carbon) and water (forming magnesium oxide and hydrogen)."

'Redox' compounds can burn underwater if shielded from cooling.
Once ignited, the gas produced aids in the shielding.
A thin water and somewhat heat resistant covering is all that's necessary.