Gas water heaters these days are pretty well insulated. If you buy the top or the line unit from any brand, it has even better insul. than the "standard" model. You can only get so good with gas, because a lot of energy goes up the flue when the burner is running, and there is significant heat loss just from convection up the flue even when it is not running.
You can add a blanket around the tank, but are prohibited from putting any blanket on top, since it would interfere with the draft hood.
Direct vent and power vent heaters have better efficiency numbers, but cost a lot more up front.
You mentioned an "enclosure". If a water heater is in any kind of enclosed space, there are very specific requirements for combustion air which you must provide for.
So, you have a few issues to look into, in advance, before you but something and commit to it.
