Marine grade is USCG approved.
Automotive grade is not usually compliant with USCG regulations for safety .... and would be in a sense a violation of your agreement to your insurance underwriter who assumes that USCG spec. materials are being used. I would think that if you had a casualty/insurance loss due to substandard hose, your insurance coverage would be at severe risk. "Marine grade" fuel hose usually has more reinforcement and integral 'armoring', and is only made (certified) from specific approved polymers, etc.