Regarding the algae, I am not worried about the blooms, I am worried about dead zones -- and water mixing seems to curtail those.

I think you under-estimate the capabilities of an oil rig. Shell's Perdido platform is currently operating at 2450m deep water. And then there are things like Maersk Venturer (which, I guess, is a ship.)

As for unpleasant working conditions, a lot of rigs are built and operated by Norwegians, in the Norwegian North Sea. I think I don't need to tell you about what conditions they are used to. Those are not Florida Men.

If you have a good solution for convincing sperm whales to do the job for us, I would be all for it. And long-term this is indeed a better solution, because not only we wouldn't need to do it, but getting their population back to what it was before we started killing them off would sequestrate something like 50 million tonnes of CO2 (Not even joking, just ran a quick calculation.) But I suspect we would need a lot of time to ramp up this process (they live as long as we do), and to convince them that, this time, we want to give them a job instead of killing them. Meanwhile, a lot of oil rigs simply sit idle and rust. I'd prefer them rusting in a nutrient-rich zone, so that they would at least feed the algae while they do.

How to pump the stuff up -- for example, by pumping down water from the surface. Or even liquified CO2, just like the other sequestration technology that relies on oil platforms (and old oil wells) proposes. It would kill two birds with one stone, so to say.