If I agreed to go to a deserted island (and this is a big if), I'd want a guidebook with color pictures of the edible and poisonous flora and fauna. I love foraging and cooking, but hate terminal food poisoning. "I wish I'd known" is too little, too late.
The second thing I'd want is a cabana set-up--beach chair, umbrella, frothy alcoholic fruit drinks (teeny umbrella optional). I want to really enjoy the midnight black sand and the soft tropical breezes. The water, of course, would be blue-green near the shore (no jellyfish), then as the depth increased, the blue would deepen to royal, punctuated by the curl of sedate waves and accrobatic air leaps of swordfish and blue marlin.
Third is Rex, my your-wish-is-my-command cabana boy. He's a master of massage, coffee, and wardrobe procurement. He lives to please me--and to write checks for my bills from his trust fund account.
Finally, I'd want unlimited Internet and email access. I'd be writing the next great American novels and my agent would insist upon it.