This is likely the answer. First of all if the ship's wifi work then they are all good as long as the satellite is the bottleneck instead of the 2.4GHz they are using. The way they layout the wifi AP along the ship you would likely reuse the same channel 1 and 11 enough to overlap and roam, maybe they can bump the power enough to cover a wider range but will make channel 6 unusable? maybe they use the frequency around channel 6 for other 2.4GHz equipment not on wifi? Maybe they have a hidden wifi on channel 6 for their own internal network?
The reason only 1,6,11 are usable is because power in the spec would encroach between 2-5 and 7-10, so if they bump the power of the AP out of spec then even 6 would be unusable.