look for the feature that indicates bottom condition, some show whether hard or soft bottom, and if it has a depth alarm. all will show if there is something, fish, under the boat, better ones may show outlines of junk on the bottom also like something to snag a anchor on.
i have just fitted one of these . i already have a simrad but another toy is always welcome.
it works very well give a reasonable clear picture and all the alarms do what they say .
only problem that i had is that sometimes the 2 sounders clash so i turn one off and back on and its ok. i think i have the transducers to close , oh by the way i have just stuck my transducer in with some white mastic on the inside of the hull