25 should be fine, its only a petrol engine of what, 2.5 to 3 litres capacity at most? I'd be more worried about how thick the insulation was on something like welding cable from a chafing point of view - the battery lead to my outboard is encased in a massive rubber armouring layer which makes it very resistant to chafing.
My jump leads at work are 35mm sq and I think they are rated up to 4 litre diesels, and diesels take much more ooomph because of having about double the compression ratio.
A Boat is a hole in the water, surrounded by fibreglass, into which you throw money...
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