My first reaction is that the switching devices for these motors sound like poor quality, resulting in spikes on switching, particularly switching off. Surge suppression is likely to cure the problem but is treating the effect, not the cause.
I use a lot of contactors for motor control and without exception use a simple fused contactor arrangement. The switch in addition to the contactor seems a waste of money and I can't see any material benefit. Design one's vendor is bullshitting.
In Europe there are probably no air break...
electricpete has covered just about everything. Something I don't specify myself, because very few end users tend to bother with them, are Rogowski Coils. Only practical for large machines, these are potentially a good method of preventive maintenance in getting signatures etc.
Two more thoughts - (i) If you might have to pull the cable, eg to the beach from the boat, over a long distance consider using hard drawn solid conductors for more strangth and smaller cross section. (ii) if you were using dc in UK waters the British Admiralty will not allow sea return as...
Bigamp should be careful of advice from manufacturers. First of all, they don't appreciate the vibration that takes place with platforms. The cables will no doubt be installed in J-tubes up the side of the jacket. Constant vibration will work harden lead and eventually it will fail due to...