I would keep it topside if at all possible, how about the tru mount like for a car, this would be solid and could mount out of the way. Next I would use the magnetic one but use a strong double side tape. You should not be able to find steel on your boat, unless it is under the hatch If they make one with this tape I would just buy that. Like said, the technology moves so fast that something could be out there. If it was me I would have a thru mount like a GPS and do it right, this way there is no chance of it coming off, unless the below deck mount is 100% now I would be weary of it, even though it would be the best place by far.