Underwater sensor networks are an important field of research. Several applications require the use of this kind of networks like tsunami or oil spill alerts. The underwater medium is very harsh and only acoustic signals can be used for the transmission of information. The use of this kind of networks is still in a developing state far from reaching standard consensus on basic aspects like carrier frequency or modulation techniques. The use of these networks for real-time applications has not been analyzed previously. In this paper we present two solutions for the scheduling of real-time messages and we provide a time constraint analysis of the performance of the network.