Noncooperative underwater acoustic (UWA) communication systems are prone to interfere with each other since the limited resource offered by the channel is not regulated by any standard. Mutual inteferences can significantly degrade the performances of such systems and it is necessary to find policies allowing UWA devices to access the same physical resource. In this paper, we consider noncooperative UWA OFDM systems competing to access the same portion of the spectrum. We show that an efficient decentralized power allocation strategy can be achieved when all the communication links apply a waterfilling policy only based on little knowledge about their environment. Numerical simulations performed with real UWA channels sounded at-sea demonstrate the relevance of our approach.