Here is another example map, this time for Generals, not ZH, which shows you how you a technique for building structures in water (or for that matter, in the air, on cliff sides, or anywhere):
MCV that builds buildings on water, slopes, rubble - CnCMaps Map Editing Forums
You will want to modify it as i made it so that the construction vehicle can only be used once (becaause someone else wants to approximate an MCV that can only build 1 structure).
