I don't need
500 feet. Part of that is just curiosity. It drops to 600 feet in front of my house and that's interesting. Deep readings are also nice for halibut fishing.
I have found that most sounders reliably read about half of what they claim if the boat is moving. My present sounder (when it worked) reads to 999 feet, but only with reliability to about 350 when moving in calm water.
The transducer is 120 kHz, which is not common. Si-Tex makes a reader only that uses a 120 transducer
. It can send out a NMEA signal that can be used with a Navman repeater station
. About $700 all in. But, like most electronics upgrades, it involves cutting new holes and running wires.
So I looked on Ebay and found a newer (still old) Datamarine Offshore sounder that fits my flush cutout. (It's not Cruising Equipment like I thought in the original post.) It goes to 1,200 feet. $100. I might get lucky and have a plug and play replacement.