Seaweed in the fuel?
Perhaps take that water and seaweed - get rid of the water and crush the seaweed under great pressure over millions of years until it turns into a gloopy liquid. The gloopy liquid can then be refined into a lighter liquid that we could put into our cars at great expense. Best to put that gloopy stuff underground preferably in volatile areas of the world so that we can be held to ransom by these areas and pressured into supporting these strange regimes or conned by our political leaders into invading some of these countries under false pretenses just to get at the gloopy stuff.
Only an idea.