See blodger's post #35 - the only thing he didn't do was the sum, which I'll do for him:
12.5 x 12.5 x 3.314 = 490.9375 square mm
If you go to a cable supplier and ask for "25mm cable" you will get something a lot smaller than what you want - it will be about 6mm in diameter (1/4 inch!)
I haven't got the cable size conversion tables to hand, but in US parlance a "500sqmm" cable is probably about a "000" cable (I'm sure someone will correct me...), and in old Imperial sizes? (I've never had to use them...). Whatever its a "figging big" cable to have to handle and terminate without the proper equipment...
WRONG - On a narrowboat you would have TWO cables between the battery and the alternator (one for the positive, and one for the negative) as you should/must never, ever, ever, ever, ever, ever, ever, ever use the boat hull as the return, which is normally the negative (This is totally unlike cars...).