So running a tube up the tether is probably possible. If you want to go the floating oil platform route, you could still eliminate the need for helium. The oil platform could have an electrolyzer/desalinator and feed hydrogen up the tube to the inflatable turbines...

Here's another problem though: when installing a turbine, the tube would have to be hooked up to both the turbine (at altitude) and the oil platform below. However, if each turbine had an integrated self-inflation unit there would be no complex fixturing needed, it would just grab on to the tether and unlock it's axle to start spinning. Later on, if a turbine needed servicing, it would just do the opposite. No hoses to attach. No valves. It should even be possible to do this by remote control, and then get towed by ship to port. In this design no oil platform would be needed, no gas cannisters to supply and collect, and the tether would be anchored directly to the ocean floor.