What's the difference between a fish finder and a depth finder for a boat? What's the best finder for $300?

i use a hummingbird 128wide. its a few years old now, but, good unit. as said, best ones, are ones that do both. map and sound the bottom, and detect fish. however, most fishfinders use a transponder, mounted to the trnasom of the boat. they only work, in water, and often, on planning hulls, at speed, they are rendered useless, due to the disurbance beneath it caused byt he hull and prop. so dont work too well as a visiual aid when at speed. need a displacement hull or a through hull trnasponder located somewhere thats alwasy in the water and free from 'dirty water' for information, the fishfinder works by detecting the swim bladder of the fish. thats how it distinguishes between a fish and drift wood or whatever. fish like mackerill, dont have a swim bladder, and so it usually wont pick them up, but, they have come a long way and very accurate now. giving size of the fish, number of them and how deep and the direction they travelling. id recommend a hummingbird, but i hear eagle are good finders. as far as gps combos though, for the price, i wouldnt rely on that too much. a good gps system is worth investing in, especially at sea, and you want to spend the money on a stand alone unit, not put all ya eggs in one basket, so to speak. Personally, i only ever by Garmin GPS systems, can spend a fortune on them, but one i have, was about 300bucks i think, but its built especially for rigid inflatables, with open decks, as it has a built in reciver and water proof, as opposed to the normal, external receivers mounted to cabin roofs etc. they give you a btter signal, but, it suited to me well. but, eagle or hummingbird, but get a seperate GPS if you want that too. thats my opinion anyway

Humminbird makes some pretty good fish finders for under $300. The only difference between a fish finder a depth sounder is that the fish finder has software that identifies fish between the boat and the lake/pond/river/sea bottom. I use a Humminbird 300sx. As well as depth and fish, it also reads the water temperature at the surface. It was around $299. Good luck fishin'.... from the North Country Redneck!