I have Atwood GC6aa-9e. Water isn't getting hot.

You didnt indicate whether the switch was defective or not. From looking at the image, it appears that the identified conductor (white) was cut. Have you tried to reconnect the wire (white to white) and see if the heater is working?
You can buy replacement parts from here:
https://pantherrvproducts.com/search.php?search_query=Atwood%20water%20heater%20switch%20§ion=product

your manual is located here
http://manuals.adventurerv.net/Atwood-Water-Heater-Service.pdf
http://manuals.adventurerv.net/Atwood-Water-Heater-Service.pdf

Try this link and open PDF file and print wiring diagram. (page 7)

http://www.atwoodmobile.com/manuals/waterheaters/MPD%2093050%20PS%204.9.09.pdf

By model number it tells me that you have Electric element, as well as gas with electronic ignition for the gas, is that correct?
Any 6 gallon Atwood will work, and if you want to have all the same features available, you simply order another 6 gal, with same features. If you decide to forget the electric element, and just go with gas and electronic ignition, they should be able to order you just that tank. You can even order a tank with manual ignition, and forget all the other options if you wanted to. Personally, if it's got hydro and electronic ignition for gas, I'd replace it with same, unless I have no access to hydro, and will never use that option. If all those options are working on yours, you can just replace the tank, rather than buy all those items new on a complete tank. Any 6 gal. Atwood will go in the hole, it just depends on what you decide you want. I don't understand what the confusion is about, but maybe I'm missing something here.

Have you tried temporarily removing the thermal cut-out from the circuit to see if ignites then? It's the little resistor looking thing inside the plastic sleeve/cover that runs to thermostat. Disconnect it from the thermostat terminal, and from wire that is on the other end, and you should be left with a female spade connector at end of that wire. Connect that directly to thermostat terminal where the other part was connected, and try it. If it works, the thermal cut out is no good and needs to be replaced for safety reasons, in case of flame out of control, it blows and shuts gas valve off.

So are you saying that the first initial lighting of the pilot, without turning to on, works o.k., good flame etc, ? If not sure, try lighting pilot only, and leave setting at pilot only, and see if it's a good flame, and get back to me.

Does yours have the little cut-out diode looking thing in the plastic jacket installed between power wire and thermostat? If yes, remove it from the circuit and connect female spade of wire, to male spade of thermostat,(just removing it to by-pass it) and try it now. Let me know what happens.

Yes. If your using it in an RV, your converter can supply the needed 12 volts, if it's not already there.

The ignitor is also a flame sensor. If it's not working to ignite, good chance it's not sensing flame to keep gas flowing as well. Could also be board.