I don't think a flex that long is made, and I doubt that you would be wise to run any kind line under a water heater. Is access under the floor a problem or are you just looking for a super easy way? You should run additional black pipe under the floor to the desired location. Unless you are fully capable of working with gas, you should call a pro. A bad waterpipe connection will just leak water, but a bad gaspipe connection could ruin you whole day.