hot water heat wire pierced?

If the wire itself is not damaged, only the insulation then you can use some heat shrink. It's like a rubber sleeve that slides over the damaged part of the wire, just heat and it will shrink around the wire.

I have no idea how you cut cut entirely through a wire with an access plate, but if you did, replace the wire with one of the same gauge and temperature rating (for example 90 deg. C). If you are at the lower access opening, the wire goes up to the upper access opening. If you are at the upper opening, the wire goes to the junction box on the top of the heater where the incoming line from the house connects. There are no other places for the wire to go. If you just cut the insulation, wrap it with 3 layers of electrical tape.

The wire likely just runs to the upper thermostat, usually you can get to the ends of every wire in a water heater through the 2 access panel's. So you'll just have to tug on it from the bottom until you identify it at the upper opening, disconnect, make your repair and return everything to proper place.

like said above, if just the insulation is damaged and not the wire even a few wraps of electrical tape will do.

If you can't pull down enough wire to fix this then go to the hardware/automotive store and buy a small can of plastic spray patch. Cover the cut area so that its no longer bare and your done. Good Luck Richard

i've had that problem to then i cleaned it and i went away , go buy a bottle of dial hand soap pour some out then filll the rest with water and clean it with that trust me it works well

Solder it back together and tape it.