Externally, Lowrance's "Structurescan" sonar is an uninspiring grey box, connected to a rather odd
looking sausage-shaped transducer.Couple it up to a power supply and to an HDS display, though, and the results are absolutely amazing -- an almost photographic representation of the hidden world underwater.
It takes a few minutes to get used to the unfamiliar
presentation of any sidescan image, but believe me, it's a lot easier
when the picture is moving than when you're looking at a stationary
screen dump. 
And out in Helsinki harbour, there was plenty to look at:
I saw the remains of an anti-submarine barrier, several wrecks -- so
clearly that we could pick out individual hatches on their decks -- and
even an abandoned anchor and chain.
Its value as a fish-hunting tool is obvious. So is its
likely appeal to divers. But even to non-fishing, non-diving folk, it's
a fascinating thing in its own right.Fundamental to its success is that it operates at a much higher frequency than a conventional sonar. This, combined with that oddly-shaped transducer, serves to focus its ultrasound transmissions into a fan shaped beam -- very narrow in the fore-and-aft direction, but sideways and downwards all the way from the surface to the seabed. In the few other sidescan sonars I have seen, that leaves a blind arc underneath the boat. Structurescan, however, fills in the gap, and gives you the option of seeing a hi-res downward-looking image alongside the sidescan picture
It's priced at £586 (but allow at least another £500 for even the smallest HDS display).
www.lowrance.com