Various implementations directed to alert zones for a marine environment. In one implementation, a marine electronics device may include a memory having a plurality of program instructions which, when executed by the sonar signal processor, cause the processor to determine one or more alert zones for a marine environment proximate to a vessel, and to receive sonar data from a transducer array disposed on the vessel, where the sonar data corresponds to the marine environment. The memory may further have program instructions which, when executed by the sonar signal processor, cause the processor to analyze the received sonar data to determine a depth line, where the depth line is a representation of one or more depths of an underwater floor of the marine environment, and to provide one or more alerts if at least a portion of the depth line is positioned within the one or more alert zones.