Due to the absence of spectrum regulation and limited bandwidth, underwater acoustic (UA) systems are prone to interfere with each other. To limit this interference, a decentralized power allocation strategy is proposed for multiple OFDM UA links sharing the same physical resource. These links are supposed to be noncooperative and aim at selfishly maximizing their own information rate. Each link is assumed to only know the statistics of its channel and the overall noise plus interference power spectral density. A game-theoretic formulation, which explicitly takes into account the random time-varying nature of the underwater acoustic channel as well as the low speed of sound, is derived. Numerical simulations show the strong benefit of the proposed approach in highly interfering channels.