If you generally always fish the same lake that isn’t mapped and you just want some information about depth and where the fish are, look for a dual beam sonar only unit.
If you like to rotate between a few mapped lakes and are mostly concerned with finding and marking what’s below the surface, look for moderately priced sonar models that offer dual and down imaging and GPS.
If fishing is your sport of choice, you may want to invest in a GPS fish finder.
Sonar technology is the most basic way to locate fish. These finders scan a wide area around your boat and tell you if there’s something down there. While they don’t provide a high level of detail, they’re better than blindly casting your line and hoping for the best.
Similar to a photocopier, a down imaging finder casts a beam straight down, scanning the area directly underneath your boat and beaming back a 2-dimensional representation of what it sees. Finders that offer dual imaging will also scan the area on the sides of your boat. These finders will provide additional details to help you reach your goal, including what depth things are at and the temperature of the water.
These more high-end units will include all of the features you get with dual and down imaging sonars, but on a bigger screen with a much higher level of detail and richer imagery. You’ll also get more information about the topography of the area that you’re fishing, to help you determine the best place to look for fish.