I'll second Hotspots, they are fairly accurate. I have the one for Pymy, depth contours are almost dead on(at least in the areas I've checked), but "fish structure" is off. I carry a Sharpie to make notes.
If anyone has an extra $1,500 you can get a side scan sonar unit.
In all honesty, I have one for work and they are pretty amazing. I've found just about any kind of structure you can think of and swimmers even show up on the screen. Screen capture is a nice component for looking at maps when you get home too.