Acoustic waves propagate slowly in water, and time-varying UACs result in inevitably high bit error rate and packet loss rate. The long propagation delay and the error-prone nature of UACs impose challenges on reliable transmissions in UANs. In this article, we identify the challenges for reliable acoustic transmissions and propose a CL-FEC scheme, which achieves opportunistic transmissions to overcome the frequent transmission failures in UACs. CL-FEC adopts fountain codes as a packet-level FEC and adopts channel codes as a bit-level FEC, to realize reliable transmissions over UACs without per-packet feedback. To further improve the throughput of CL-FEC, we formulate the transmissions over UACs into a stochastic throughput optimization problem. A discrete stochastic approximation based algorithm is then developed to achieve the optimal CL-FEC by online exploiting channel estimating and algorithm iterations. Simulation results show the asymptotic convergence and the iterative optimality of the algorithm.