Waterproof case for the IPOD - $20 http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=400243&is=REG&addedTroughType=search
Rig it up right along side your graph on the bow in a holder of some sort. Run the cable down your trolling motor shaft. Viola, $210 Biosonix. You could go way cheaper on the MP3 player component too. I imagine 512mb would be plenty of space for fish sounds.