To add to what Joe said:
The term "Fish Finder" is misnomer. Can they really show you where the fish are? The answer to that is "How much do you want to spend?" Joe's stated purpose is spot on and is the best way to use any sounder -- to look for structure, rocks, reefs, etc. While my GPS is extremely accurate, the value a sounder adds is to show you exactly when you're approaching or on top of a reef or other structure. You can also see bottom composition on most sounders - soft strata (sand, muck, etc.) appears as one color (usually a light blue) while hard bottom (rock, reef) appears in a different color (usually red). As far as showing you individual fish down there? Well... perhaps, but it's not something you can rely on to any degree. Some sounders will show arches to indicate a fish, while others will show you a fish icon, size based on the degree of return signal from supposed fish. My sounder (a Furuno unit) will show bait schools suspended at various depths and often indicate individual or groups of fish hanging near the bottom, even on the deeper reefs to 300 ft. or so. The smaller sounders like Joe's are not as powerful, but do a fine job in shallower water. Well worth the investment of $100 or so for a kayak or smaller aluminum boat. The biggest advantage you get is not wasting valuable fishing time on non-productive areas, i.e., sandy bottoms. Unless of course, you LIKE catching lizardfish!
Like any piece of electronics, it's only valuable if you take the time to read the manual, go out and use it, and learn to interpret the information it's showing you. Experiment with different ranges and modes, if yours has those features. Commonly, bottom lock is a feature on most sounders that is valuable because it keeps the beam focus on the bottom. You may also have a zoom feature which amplify and enlarge the picture for you. If you have the ability to change frequencies (commonly 50khz and 200khz), make sure you try both those modes to.
Final piece of advice? Unless your transducer is mounted in the middle of your kayak/boat, keep in mind that what you're seeing on the display you've already passed over! Mine's on the stern of my boat, so this is very true, given speed and processing time. Turn around and go back over the object to relocate it, then mark it on the GPS. Have fun!
