I'd say the body of water and here's why - you'll probably be able to find fish in the water, hydration from the water and if you have something to row with which should be somewhat easy with boards and stuff like that floating on the water, you can paddle yourself in a particular direction. If you see any signs of land, I'd paddle towards it. In the desert, you're stuck with ridiculous which in itself can kill you, venomous animals such as snakes, scorpions and other dangerous criters and on top of that, your only source of water cutting a cactus (good luck trying to find something to do that). What would you choose?