The reason is because you have created a parking spot at a fixed height. If you remove the parking spot, runway and taxiways, it will sit at the water level:the aircraft is STILL sitting on top of a raised hard surface which is from what I can see the taxiway???
Hmm, in the original thread on water airfields, the author mentioned that he found variation in ground height when using TCalcX. However, the variation isn't present in slew mode, only in normal mode.I noticed the swell will create a slight step up if the runway is sitting slightly higher then the lowest swell.