To get a negative pressure on the WH would require the water flow out to be greater than the well output AND the WH would have to be higher in elevation than the water discharge point. Marathons come with a vacuum breaker so if negative pressure happens at the WH, air is admitted into the WH. Without the VB (or the VB fails to operate) the polybutylene inner lining might be peeled away from the fiberglass tank. This could lead to failure I suppose. Example: You could have a wooden barrel which would not hold water. If you line the inside with plastic it would now hold water. The plastic is the water tight part and the wood provides the strength. If the plastic is not kept against the wood if would not support the water.