A supertanker of that size, if used to store the diluted oil/water mixture on the ocean surface, would fill up in slightly less than two minutes, assuming a collection system that could suck up water over an area measured in square kilometers. You'd need 730 supertankers to store each day's worth and the treatment problem would still be there.

The problem with treating surface water is that the oil is hugely diluted with seawater and the sheer volume becomes too big to handle. What you want to do is get at the oil before it's diluted so much, at or near the source. That's why there's this big effort to control the seafloor source by dropping a containment chamber or two on top of the wellhead and/or collapsed pipe and pumping from containment. You might still get some seawater dilution but it's going to be orders of magnitude less and the separation problem becomes manageable.

The other way to go is to drill a relief well and pump out of that. I understand that's in the works but it will take months to complete.

The last resort is to plug the existing well but I'm pretty skeptical about the permanence of the sort of plug you could put in under the conditions. You might get something that would slow the flow rate but it probably wouldn't last long before blowing out in turn. Plugging could help until the relief well is in place though.

The BP management and the operations people who caused this mess are deserving of lots of blame and finger pointing but there's no doubt that the best engineers available are on the case now and are looking at anything remotely practical to stop the leak. I'd bet any reasonable amount that they did parallels to my calculations within the first few hours after the blowout, dismissed large-scale surface treatment as an option and commenced work on real-world potential solutions.