Take a look at my aircraft carrier script - basically you place an object in the water where you want it, then add a call to the script from the init line which spawns the vehicles.
Becuase the LHD aircraft carrier is a building to the game - you could easily swap the parts, recode the script to make it spawn say, a bunch of concrete blocks in a square half above sea level (half in water half out), with anything you wanted on top of them (I don't know, make it an oil rig?)
I'm interested if you do this, I just found that nice LHD script and made it work for dayz. It'd be cool to have an oil rig, maybe a bunch of docks connected to make some type of survivor camp on water.
Things to note - game still thinks you're over water - so it'll get cold. Put some firebarrels around to heat things up.
Vehicles fall through - I can't (yet) get vehicles to stay on top of buildings (any way I spawn buildings in) - the vehicles spawn first, so the start to fall, then the building spawns in and the vehicle has fallen a little into the building so it explodes.