Look here.

http://www.spe.org/jpt/2009/03/subsea-water-treatment-injection-technology-promises-production-gains/

If you are down 2000 feet, then running electricity is expensive, but you can pump water down an umbilical. At the bottom, they add chemicals, and pump the liquid into the well. The chemicals often include a little grit to enter the frac pores and keep them open. In addition, they can use the hydraulic power of the water they pump down to run a hydraulic ram with a large bore pumping the fluid using a small bore. so a 10" diameter ram feeding a 1" diameter ram will amplify the pressure 100 times. So if they pump down 200 psi water, it will be 200 psi water at the bottom and acting via the 10" ram it will create water of 200 x 100 = 20,000 psi. This might suffice to frac it.

I see they say there is only a power connection in their system, so they might not use water from the topside?

Since they are in international waters, there is little patent protection or enforcement capability, so they keep things secret, which is why you have no access.