NASA, while developing the H2 fuel tank for the X-33 project, finally discovered a trick used decades ago by the surf board industry.
Maybe a honeycomb board has other advantages as well.
A 15 lb surfboard made of aluminum tubing would displace 2.5 ft^3 and could be safely pressurized to 120 psi.
The air powers an air tool which powers a high volume - low head pump for a jet ski with high propulsion efficiency at lower speeds (< 10 knots), the same high bypass idea as in those newer aircraft engines where the fan is as big as the fuselage.
If you are prepositioned well a large wave takeoff requires 5 - 8 hp for 6 seconds so you get one or two assists before you head back to the air compressor.
Bret Cahill