Yes, they had a tough time of it; they needed to empower themselves with a bit of survival knowledge and maybe a few more survival tools.

With the clear part of the sails and/or plastic, they could have made a solar water generator. By digging a hole near the beach and draping in the plastic, condensation collects on the interior and will run down into a centalized receptacle. Correct me if I'm wrong, but I believe you can generate water from salt water by using a solar generator. If not they could have mashed the cactus and distilled the pulp for plenty of water.

Also, had they dug a hole in the ground a foot or two deep, their bodies could have enjoyed cooler ground; cover yourself with sand and they wouldn't have been so badly eaten by the flies. With added water and sleep they could have had the energy to search for food and would have probably survived quite a long time. Their four day stint was pretty desperate.