Bury the oceans?
Hi Nick
There is the possibility that the oceans are being dragged into the mantle anyway. Within a half billion years Earth will be dry even without the Sun's brightening.
Personally I think we can do better - eventually we will alter the Sun's evolution, overturn the Core, and drag out its life to 50 billion years or more. Eventually we'll catalyse proton decay in the Solar corpse and extend its span to 10 trillion years or so. Or channel WIMPs and burn those for ~ 10^20 years.
But I'm an optimist.