Hi John,
While saving my coin to get a 598, I downloaded the 598 Owner's Manual, printed, and read it all. Also, subscribed to this site and the other Humminbird site and read everything. I came across a Humminbird Greg post where it said you could upgrade to the HDSI ducer by trading in your new, unused compact SI ducer along with an upgrade fee. As soon as I opened the box and registered, I got hold of HB Customer Service. Within 5 days, I had the HDSI and went out to record my project (attached screenshots).
From other posts with bigger units, I think mine are just as sharp from a 598. These pics were from a lake with a high load of suspended silt and clay from Mt. Rainier glacial runnoff and looks "milky". The 598 with the HDSI and 4K watts did well, even to 150+.
Lou