BrewLAN is a mod for Supreme Commander: Forged Alliance that has been in on and off development by a team of one for almost a decade. It adds a wide variety of new units for all factions, as well as offering several new game modes and features. The core design goals for the mod are simple; to add new, interesting, and visually aesthetic units which complement those already in existence and to maximise technical compatibility with other mods.
New feature to the Waterlag sub-mod: It now adds legs underneath buildings that it allowed to go on water, and don't already have floatation devices modelled into them.
This is done dynamically, and currently only applies to UEF and Cybran buildings. Cybran have three different sizes of leg model, and UEF have two. So far.
Is that a Cybran T3 Heavy Artillery ON WATER? AWESOME!
Some Seraphim and Aeon structures would become..amphibious?
Aeon and Seraphim things currently float without any justification for their floating. I haven't made them any legs yet.
Aeon/Seraphim use some type of magnetic or gravitic levitation I believe. Cybran UEF could also use pontoons/barges aside from a oil derrick type base.
Nor only do I expect them too, they exist today. Oil derricks that are moveable from site to site are virtually exactly what you describe. Only some are mounted on pillars on the bottom, where the water is shallow it is practical. The famous disaster on Piper Alpha in the North Sea was a shallow water rig (mounted on pilings).
Google.com
In deep water, you have ones like that BP rig Deepwater Horizon that blew up in the gulf. Notice the picture in the wiki entry of the unit in water, and on the transport ship. Only a fraction of its overall height is submerged, but that fraction contains a lot of volume to provide the buoyancy.
En.wikipedia.org
I'd note also, that these rigs sit in the Gulf of Mexico and have hurricanes go directly over them. Yet each hurricane I don't hear of them blown away or sank. If you look at some of the photos of the things they are massive, so even huge storm waves they simply ride out.