Vasuda was mostly desert, yes, but they did have some surface water (or else how would anything have survived?), and there were even a few small seas. The shores of these seas became the geographical base for all the major Vasudan population centres. Being on the whole a wiser and more philosphical people than the Terrans, they did their utmost to preserve these rare and valuable seas from pollution, exploitation and abuse, which is why fish are such beloved creatures (and prized delicacies) to the Vasudans.
Just as the Terrans were inspired by birds, who could fly while they were rooted to the ground, the Vasudans drew their inspiration from the fish. Their space vessel models reflect the innate Vasudan desire to "swim through space," as they would say.
Alright, I made that up, but it sounds good, doesn't it?

Edit: I am an excellent typoist.