Al Gore claimed in an Inconvenient Truth that one reason melting ice caps was such a problem was that the ice reflects 90% of sunlight, while the open ocean absorbs 90% of the sunlight.
Open ocean therefore provides a better, longer-lasting, and deeper heat trap than just the greenhouse gasses in
the atmosphere alone. Furthermore, hotter oceans melt floating ice from below, and can cause coastal ice to receed from the coast. What's more, in tropical regions, hotter water makes for more powerful hurricanes (AKA typhoons, AKA cyclones, etc. depending on your part of the world.)
Sadly, with temperatures going up, and what with water being hard to keep ice on, there's little chance of getting that ice back soon. But without something reflective up there, the problem will just get worse.
Never fear. We have the solution right here, and it is... MYLAR. As evidenced by the whale-swallowing ballon problem, we already know that mylar is cheap, reflective, and it floats on sea water. So, huge sheets of mylar can be floated in various parts of the ocean, thus reflecting the sun's rays before they enter the ocean.
With a product that costs mere pennies to the square mile, we can increase reflection at the poles, keeping the polar waters cooler, and helping to prevent further loss of ice sheets. We can increase reflection around the gulf of mexico, and other near-coastal regions to reduce coastal water temperatures, thus cutting out the power for cyclonic weather patterns before they hit dry land. We can put mirrors out around tropical waters, thus reducing hot spots, and making the ocean safer for coral reefs, and thereby protecting our fishing industries. we can put these on top of the ponds that form when ice sheets start to melt in the middle, and hopefully re-freeze, or at least slow further melting there.
Won't these huge sheets break up in the waves? Probably not, as a matter of fact. Waves only break near the shoreline. In the deep ocean, it is just swells... OK, so it can get pretty rough when there is a storm, but these should be pretty cheap to make, and fairly easy to deploy.
What if ice forms on them up in the arctic, and they sink? Well now... ice floats, and ice reflects, and ice is what we were trying to replace with these in the first place, so if ice forms, and stays, then we won't be needing these any more, so they did their job.
What if polar bears, or whales, or some other critters rip them up? Uh... yeah. They're cheap. Sadly, animals might get tangled in the debris, but if we're really talking about the fate of the earth here, maybe we can afford to risk losing a few animals. if not, we can let things get worse. Supposedly we're also killing our oceans, so there might not be any polar bears or whales to worry about in the future.
OK, but who's going to pay for this? Ah. Good question. Well... anybody might. However, let me propose an additional feature: Big reflective things in the ocean like this could be shaped to form... company logos visible from space, for example. Know that Google logo on the top of their building? Now imagine that... only it's saving the earth, and it's a few miles on each side. I'd call that good publicity.