Reading this thread with some interest. I have had this discussion re the aircraft/Ekranoplan/ boat dispute many times and I think if anyone is in any doubt, common sense should prevail. Firstly, what is an aircraft in the established sense of the word... it is a construction/machine capable of flight and three dimensional movement ie two horizontal movements and one vertical movement. This ensures it is in a different category to all other craft as it is not tied or limited to surface movement. The bottom line is 'what goes up, must come down' and in the latter case can kill someone... not so in the case of surface skimmers or boats. It is for this reason aircraft occupy a specific frequency which should remain free of all other interpretations.
When aeromodellers want to go flying, they will always seek out their like minded collegues to ensure they are A: not going to destroy their aeroplane and B: not going to lose control through interference and injure someone.
In the case of surface skimmers, although they do rise in the air medium, they are constrained to operating on a cushion of air or 'ground effect' and are therefore not true aircraft. The design of these machines means they are incapable of soaring into the air if they are correctly designed, in other words incapable of 'free flight'.
My advice would be to treat free flight, soaring, and climbing machines as aircraft and that means 35mHz and everything else tied to the surface is going to be 40mHz or 27mHz. Of course, the newer high frequency equipment has its own applications already allocated but just obey the rules and you can't really go wrong.
If some unforseen accident were to occur and one was taken to court, I'm sure a good lawyer could argue that the 'offender was seen to take all 'reasonable' steps to ensure common sense and safety were applied. Beyond that, there is not much else you could be seen to do.
Good luck with the airboat/ skimmer but make sure it isn't on 35mHz!
PS, all the 'off frequency' toys pouring into the shops should be fed through the crusher!