It was me.
I'll try again.
It has nothing to do with connecting the bonding to the converter.
Non-isolated converter means that inside the converter, the 24 volt input negative is connected to the 12 volt output negative.
Inside the ICOM SSB, the 12 volt negative is connected to the SSB ground point.
ICOM recommend you connect the SSB ground point, and the Tuner ground point, to the radio grounding plate on the skeg (which is in the sea water).
With your non-isolated converter you now have an electrical path from the 24 volt negative to the immersed radio grounding plate and through the salt water to the anodes on the rudder which are connected to the ship's ground bonding. So you have a path from the 24 Volt negative to the ship's ground, which in Amel land is not allowed.
Two ways you can deal with this
1. Use an isolated converter. The 24 volt input negative is isolated from (not connected to) the 12 volt output negative, so the fact that inside the SSB the 12 volt negative is connected to the SSB ground is no issue since there is no connection to the 24 volt negative.
2. with a non-isolated converter, install a relay in the 24 volt negative feed to the non-isolated converter, powered from a switch (or the existing cct breaker in the hanging locker, but that's a pain to reach) which you use to switch the 24 volt positive on to your converter and REMEMBER to switch it OFF when not using the SSB. This will connect the 24 volt negative to ships ground, as discussed above, BUT only while you are using the SSB. The short time won't cause any corrosion issues.