On Thu, 3 Mar 2011 10:48:14 -0800 (PST), Frogwatch
wrote:

Would this work?
Put a water soaked blanket on the deck (or over the cabin). It would
cool the surface below by evaporation in sunlight. When the water
evaporated, you'd pour more water onto it. If salt water, you'd have
to pour enough to remove the salt left behind by evaporation.

Your best bet is to use enough shades and awnings to keep sunlight
from reaching the deck and cabin top in the first place.