being a huge star wars fan since before I could make my own bowl of cereal, I dont have any problem at all with the poster. The stormtroopers are not walkng on water, you can see the body of water (river?) is very small, and you can see both banks of the 'river'. Seeing both banks, its safe to assume that the 'river' isnt deep at all, and the stromtroopers are wading thru it. I think i even see white-water splash at the feet of some of them.
Considering how the death star pertains to the story, i think its apropos that the death star be 'looming' over the entire scene because its fate rests in the outcome. (the stealing of the DS plans)