Fundamental differences between underwater acoustic propagation and terrestrial radio propagation impose the design of new networking protocols. In this paper, a multichannel MAC protocol, MC-UWMAC, especially designed for underwater acoustic sensor networks, is proposed and evaluated. MC-UWMAC is a low power MAC protocol operating on multichannel using a single slotted control channel and multiple data channels. To guarantee a collision free communication, MC-UWMAC uses a virtual grid based slot assignment linked with a quorum based data channel allocation. Specifically, control channel slots are dedicated for handshaking. Data transmission takes place in a unique data channel especially reserved for each communicating pair. Simulation results show that MC-UWMAC can greatly improve the network performance especially in terms of energy consumption, packet delivery ratio and end-to-end delay.