[dmix]

Those "floating" wind turbines are still bolted to the seabed (so they don't float away) and are interconnected with wires. They showed how they work via infographics on their website that was on the HN frontpage recently. So the OP's comment about the sloping sea shelf is still relevant.

[jerven]

Not as much as before, because the cost of cable to hold down the turbine is the only thing that gets more expensive with deeper water (the part connecting the floater to the "bolt"). These cables are relatively cheap.

The other option's jackets or monopiles (95% of the market) require almost a 500 kg of steel extra per meter (very rough guideline). This means that at depths more than 25meter the monopile/jacket is more expensive than the wind turbine on top. With the hywind style floaters, this is not the case.