I know otters blow bubbles into their fur to isolate themselves, but this also makes them extra buoyant. For the babies this is quite handy (less drowning etc) but for adults idk if it really helps.
It really depends on the fur texture probably, seals are also coated in fur but streamlined to perfection.
The atmosphere sure changed a lot because of life, which might have had its effects on incoming solar radiation? Which might have changed the temperatures of some ocean currents/continental plates? I don't think it would differ significantly