In the first level, go to the northernmost island, with the "HELP" sign on the beach that Decay found. Stand on the campfire near the wreckage with the "HELP" sign facing southeast. While facing southeast, walk up the hill and into the shade. Continue walking until you reach the top of the hill. Then, face northeast and walk through the brush to find an area where the sun is shining brightly. Look at that location to find Wilson, the volleyball from Castaway.
Feral Attack in the early levels
Enter FeralAttack as password.
Restore Health
Enter ImJackCarver as password.
Titanic
In the first level, go to the northernmost island, with the "HELP" sign on the beach that Decay found. Go to the eastern part of the island, and you should see a large spire of rock when looking east. Swim to it and continue until you are on the east side of the rock. Swim to the bottom of the water, then swim slightly farther to the east and you should find the sunken Titanic. In the Titanic are two assault rifles.
Castaway reference...