i know this thread is old, but, I FOUND THE ANSWER! get GeniEd2, and open up the files it says to, go to the bottom of the list, and find sea gates, sea tower, and sea wall, and unselect 'hide from editor'! then go to tools and select copy to all civs, and save it. go in the map editor, and go to buildings, then look down the list, and it will be there! they go in the water, and the towers shoot! its so awesome!
[Edited by krazy_killer, 8/23/2009 7:42:36 PM]