Have you given any thought to, or tried this? Copied post from BBC.

The program Dr. Depth will allow a direct connection to get the bottom hardness live. It transmits the same data as in the slg or sl2 files via Ethernet. You can also play around with the Ethernet packets and make your own software for it. I am sure if you wanted to put enough effort into it you could make an interface between the live data and the sonar log viewer to view live data on your PC.

You can make your own cable reather easily if you have one of those connectors to either make an RJ45 connection or a fast connect RJ45 cable end:

1.Get a Lowrance Ethernet cable.
2.Cut off one end.
3.Strip the wires and connect to an RJ45 this way:
Pin on RJ45 - color on Lowrance cable
1: white/blue
2: blue
3: white/orange
4: (leave open)
5: (leave open)
6: orange
7: (leave open)
8: (leave open)
You do not need to connect the shield wire.

You will need to disable the Windows Firewall.