You will only get icing (with the exception of hoar) in visible moisture. Essentially, you will get the most serious icing in cumuliform clouds, so you are looking at staying out of convecting air. Will it convect over water or land? Depends where in the World you are and what time of year and day it is.
If you include orographic convection in the equation then I suppose you would want to stay away from hills and hence the land, but that seems a tad tenuous to me.
If you consider what actions you would need to take if you experience heavy icing, then I suppose over sea is better because you can descend lower to warmer air.
If you are thinking of warm front icing then it makes no difference, because the theoretically best action is to climb into the warm air.
If the choice is the mid Atlantic or the Central Asia then there would obviously be less moisture over the huge land mass. But that seems to me to be a stupid comparison.
Like you say, seems very weak to me.
Any body else with any ideas?