It is for on a boat with everything on the boat is grounded to the same point on a 12v battery. The UC is only going to see SCL and SDA.
I have designed it to fit on a UM72 din mount like this:
Narrow the +12V buss a bit, it is very close to the relay contacts (remove the green).
Suggest you place a power LED on the PCB too.
Where are your mounting holes 
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Good suggestion. I was thinking it was a bit close. Many designs call for cut outs for isolation but they are designed for 240v on one side and low voltage DC on the other. I was wondering if I would get away with it. I should probably use a copper fill to be more accurate but have not managed to get easyeda to do that well
No mounting holes yet. Mounting is on the DIN mount. I will put some holes also for options
Have you considered only one fuse on +12V, the other fuses would then be redundant ? 
The fuses will be to protect the wires running to individual loads. I should maybe fuse the main unit also. On boats it is not unusual for a short in the 12v system and long wires can get very hot
Yes.
BTW, where is the terminal for +12V ?
The terminal will be a buzz bar attached somehow, maybe with screw standoffs. I have not decided exactly yet. Might solder wires to pads at common relay points and take to a buzz bar or somehow use the standoffs to conduct
That's bus bar. 
I often place a 1N4007 from +12V to GND (reversed biased).
If you were drinking the previous night you might connect the 12V reverse polarity, the diode would blow the main 12V fuse.
It makes a buzz! 
I looked into reverse polarity protection but since it will be a positive bus 
bar system it would be odd to attach the negative to it. Something to think about though
Not sure the fuses oval hole can be managed easily by the manufacturer.
I took the fuse from the JLCPCB library. I will double check
But . . . . . one 1N4007 is so easy to add to the PCB 
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I will cover the bus bar in red shrink tube! 
NO !
Use clear UV glue/resin 
I always duplicate high current traces on the other side of the board.
Leo..
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Great minds
Well, if you have a double sided board, you would certainly be silly not to.
I see blue traces on the board.
Leo..