Thanks for all your responses guys. Very impressed with the images from the Humminbird, but most of the wrecks around here don't start until 35m so not much use.
I have all the lats/longs for the wrecks thanks to a very freindly angler (who's boat needed fixing
), and from other divers, skippers in the area. Plus I have pinged a few myself over the last year so actually finding the wrecks isn't an issue.
Since my original post my RIB has shrunk so a towfish setup would not be practical anymore. Is the Lowrance unit sidescan? and can I put the images onto my computer at home? As I would like some side scan images for a small film project I am doing next year.