Even if lithium-ion battery-powered cars were as safe as gasoline powered ones, I can imagine the significant weight of electric cars is a major issue still.
I can't imagine it makes absolutely any difference to a car ferry. And also it's not like all EVs weigh more than ICE cars - we have an EV that weighs 1200kg and a normal ICE car that weighs 2200kg - why is the first one forbidden but the other one isn't?
Norwegian coastal ferries transport just a few dozen cars, alongside 600+ passengers in coastal mode, with 1100 tonnes DWT. The car weight is not really material. If it was, they'd put a weight limit on the service not a nature limit.