For an example of the method of building buildings on land that can then move out onto water, see the floating naval shipyard in my navalised map.ini :
Beng's Naval Mod Map.ini for Zero Hour - CnCMaps Map Editing Forums
Basically the shipyard object has physicsbehavior, AIupdateInterface, and Locomotor, enabling it to move. It's locomotor set NORMAL includes an amphibious locomotor and it's Draw module calls particle effects to make ripples and wakes when it is MOVING OVER_WATER. After beiing built on land it can be moved to water, and then i made it such that an upgraded is needed to give it the command set to build boats, and the same upgrade triggers a locomotorsetupgrade which makes it no longer able to move, thus fixing it in position. This is not necessary for functionality as it can build units whilst on the move, but i did this to make it easier for enemy players to target your shipyard (since it can't run away), otherwise it becomes hard to play against people who have moving buildings.