This paper investigates the MAC protocol tailored for wireless sensor networks deployed in the intertidal environment (IT-WSNs) where the ebb and flow of the tide influence the packet transmission. Most packets fail to be delivered when the sensor node is submerged by seawater during high tide. To deal with it, an environment-aware electrode module is designed to detect the tide level and determine whether the sensor node is underwater or above water. Based on the electrode module, this paper proposes an environment-aware MAC (En-MAC) protocol. En-MAC combines the electrode module and packet delivery ratio (PDR) to determine the transmission strategy, so as to reduce the number of retransmissions according to the environment. Both simulations and field tests are implemented and the results demonstrate that En-MAC performs effectively and efficiently in IT-WSNs.