Ok, props to Biosonix, they invented something totally cool and the consensus is that it works. No, I don't own one but I sure would like to.

The thing that kills me is... why the giant box? Why not:

2 gig Ipod Nano - $150
http://www.newegg.com/Product/Product.asp?Item=N82E16855101430

Underwater speaker - $40
http://cgi.ebay.com/DARAVOC-Underwater-Speaker_W0QQitemZ5852375633QQihZ008QQcategoryZ14995QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

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.

I'm just saying.