do you mean the main battery cables? If so i used i think 25 square mm cable from a welding supplier, (centre core about 8mm dia) its nice and flexible and cheap, also available in other cross sections too. if you can get the stuff thats tinned copper its a bit more seawater sesistant.
As long as the physical size of the cable isnt a problem then the larger the better in terms of voltage lost. (thinks back, My PTT was slow until i went to 25mm cable)
The other alternative is SOME jump leads use good thick cable which would be suitable, beware of cheap ones which are all insulation!
In reality a 115 yam won't take much cranking current, so you wont melt even thinnish cable but the voltage drop is more relavant because the cables will be a total length of 12 feet! Take a look at 15mm and 25mm cable and use the thickest one that will fit through the powerhead grommets.
crimp the ends on and solder if you have the facility to do so, then wrap in self amalganating tape to keep the sea out.
I am usually not as green as i am cabbage looking.