hey i don't know if this is the right price for you but i picked up a broucher on this unit not long ago. it's called the Smartcast wireless fishfinder, model RF10 . it's got a mobile station & a remote sonar sensor. you keep the screen with you and attach the sensor to your line for example while in canoe and just cast out and retrieve line as normal and read the screen as you wind back in. it says it has a range of 33m (100ft) radius including depth. it looked alright to me although haven't heard anymore about them. just another idea to think about.
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the sensor is about the size of an egg and can also attach a line to it and place a hook or lure from it so you don't waste a cast. i think it was around $350-400