amphibious transport

[MRMIdAS]

AFAIK the "shore" be it beach, grass or whatever, is counted under "water that all units can walk/drive on" and unless a unit is fully amphibious, you can't load units on a "shore" piece.

I'm assuming it's somthing complicated like "well X unit is restricted to water, Y unit is restricted to sea, these units should never meet on the same tile" and may be connected to the absolute nightmare it is to get boats to attack vehicles on land.