If you're fishing from a boat then fish finders can be extremely useful.
Remember that only the most expensive ones can be truly called fish finders, cheaper ones will pick out shoals of baitfish but are unlikly to be able to pick out individual fish.
Perhaps their best feature is in telling you how deep the water is and what structure is down there i.e pinnacles, plateaus, drop offs and weed beds.
It's possible to buy fish finders that you cast out on a rod and line, I wouldn't bother with these though, they seem like a waste of money.
I couldnt agree more and feel this is a very accurate assesment of FF's.
I have a 'Cuda 242 mounted on my kayak that I paid a whopping 38.00 fun dollars for on clearence sale at Wally World while considering a signifacantly more expensive model. It works out great for me. It provides such a wealth of info that its hard to imagine hitting the water without it. I would spend the money on a considerably better FF if the possibility of "Turtling" wasnt so high or if I planned to never go BTB's on the coast. However I am really not as interested in finding specific fish as much as I want water depth, Temp, see structure and bottom features.
Here is the one installed on my Prowler 13 from Ocean Kayak...
Mine does a pretty good job of actually picking out fish. It does take a little experience and skill to distinguish between bait fish and a school of suspended bass or crappie, but with practice you can get pretty good at it. Im not as hung up on distinguishing what type of fish I am seeing as I am about thier orientation...ie are they suspended 4 feet over cover in 20 ft of water as this will give me a good starting point for actually finding where the fish are going to likely be and what lures I will need to use to get to them.
As previously mentioned, the fact that even cheap FF's will give you depth, temp and point out bottom features are likely the most important aspect. Without this knowledge you are basically shooting in the dark. Knowing where these features are will allow one to eliminate a lot of unproductive water in short order. Knowing the water temp will also help one in locating fish too as well. Most will also allow you to determine the compisition of the bottom and if it is Sandy, Rocky or somft much bottom. This info alone can often prove important during the spring or species you are chasing.
Another aspect of choosing a FF is the transducer that comes with it. Some will use a 20 degree cone which give a slightly more accurate, but smaller picture of whats below it. This is also good to use in deep water thats deeper that 60ft. In shallower waters a 60 degree transducer will give a wider veiw of whats below it.
I have seen and used the models that you attach to your line and cast out and reel it back in. They are very marginal performance waise and dont provide near the detail one really needs unless your already familar with the area you are fishing and what it looks like under water. They are very rudimentry at best and in my opinon not worth the out lay of fun dollars.
One feature I now wish I had now that I am doing some fishing in costal marshes is a FF with a GPS and one that has a MOB (man over board) feature. This makes locating previous bottom features much easier and the MOB feature can be activated and used to retrace you way back out out a secluded marsh or water way where if your sitting low to the water in unfamilar waters where land marks cant be seen is indespinsible. FF's that have these two features are considerably more expensive and the better ones require that you buy a chip/card to install that covers the areas you are in for the best detail of the area your fishing. These chips often retail for a 100.00 or more dollars per a chip. Storms and wave action and the associated errosion can result in these chips to be updated periodically to keep up to date as changes occur. This may or may not be a worth while feature worthy of the added cost.
Im sure I could add several more paragraphs about this subject but this should give you some basic considerations.
