Insulation will only slow down the heat loss. If the air temperature in the crawl space goes below freezing, the water inside the water heater, and in all the water pipes, will freeze. If the house is left heated while you are gone, the air in the crawl space may not go below freezing.

But if you are trying to save on heating costs while you are gone, then you want to turn down the house heat to about 40-45F, and that may allow the air temp in the crawl space to go below freezing. You could go into the crawl space and turn the water heater down to its minimum temp before you leave each time, but that will a pain in the rear, unless you have an unusually high crawl space. Also the minimum temp on the water heater could be in necessarily high.

My suggestion is heat tape. Ideally you would remove the outer tank cover and insulation, wrap heat tape around the water heater, reinstall the insulation and the outer tank cover. You might be able to snake the heat tape under the insulation in places such as down the side between the access covers, around the bottom and around the top, this would work just about as well.

If you have to put the heat tape on the outside, then you need to cover the whole tank with another layer of insulation to force the heat from the heat tape to go inward to the tank and not just radiate into the crawl space. While you are at it, run a little heat tape along the pipes and insulate over them as well.

Now you should be able to turn off the water heater and turn down the house heat. The heat tape only heats to about 40F or so, but it doesn't use much electricity either. Be sure to turn off the main water supply as well and open the highest faucet in teh house, just in case something goes wrong.