Please refer to:
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In this document you will find a reference to the needed ground in RS-485 systems. While many people will tell about their successful system that did not use a ground, I can tell you about damaged transceiver chips in systems that did not include the ground. Perhaps you will not see the problem on a boat. Where I experienced the problem was RS-485 communication between buildings. The cabling had to be run by union workers which were lazy. They gave me onto four conductors for my full duplex system. Without a common ground between the RS-485 chips, they had to rely on local earth ground which can have a large offset. We had repeated intermittent transciever failures, often after power problems or lightning storms. After replacing the chips, it would work for a few weeks and then fail again. After researching the transceiver chips and adding the ground wire, the problems went away. No more damaged chips because both sending and receiving devices were working from the same potential.
My suggestion would be to contact the manufacturer of your transciever chips and ask the grounding question very directly. Many manufacturers show simplified diagrams that do not include the ground.
Added after 8 minutes:
With regard to the bridge power supply, I would remove the 1N4007 diode. This is a crowbar diode that will blow the fuse when polarity is reversed. Since you are using a bridge rectifier and want to be able to run with either polarity, this diode only causes problems. Again, the diode's purpose is to protect downstream electronics from polarity reversal by blowing the fuse.