all the metal parts of your AMEL 55 (including your fuel tank) are connected together with the green/yellow wiring net and with the zinc anodes of your rudder EXCEPT the SSB ground-plates, tuner and transceiver.
DON'T CONNECT the SSB ground plates to anything else, especially the general zinc ground circuit !!!
You would then send a lot of power to all these metal parts while transmitting (moreover if you speak loud!).
The HF-SSB radio system must absolutely be isolated from anything else, including yourself (don't touch the antenna while trasmitting).
Moreover, connecting to something which is not in contact with water (your fuel tank) would not dispatch more power (the purpose of the ground plates is to dispatch the excess of power while transmitting).
- check all your connections on the SSB system
-clean the ground plates thoroughly (with some acid stuff in order to remove the scaling)
-check the continuity (0 Ohm) between the ground plates and the copper strap in the lazarette and the transceiver ground terminal
-check the good contact on your whip antenna
Good luck and be prudent with electrics.