The EU and USA have both produced new regulations for electric drives in boats, and part of those regulations are associated with the safe connection of the batteries. I'm not going to go into the regulations, but what you see is the cable for connecting between batteries - the diameter is approximately 13mm and pretty difficult to bend. Because it's marine cable the copper strands are all tinned. Because of the voltage the cable is double insulated and fireproof.
The cable that connects the batteries to the buss bars that connect the motor are even more complex, with armoured screening too...
