The Whale Skeleton is the remains of a whale located to the west, off the shore, of Banham Canyon in Grand Theft Auto V. It consists of the large skeletal remains of a whale, which can be found at the bottom of the ocean.
There's also another identical skeleton just west of Fort Zancudo, slightly further away from the cargo plane wreck.
Off of the west coast of Banham Canyon, the skeleton of a whale can be found on the sea floor, partially embedded into the sand. Players initially believed that the skeleton was actually that of a sea monster, or some other fictitious sea creature, but it has been proven that the skeleton is from a whale.
The only way to see the whale skeleton is to scuba dive down to the sea floor, or use the Submersible. The whale's skeleton closely resembles that of a Humpback Whale, which is common along the California coast. Another skeleton can be found on the East coast, near the Seashark race.
It is proven that this skeleton represents gratitude towards players that search and hunt out of the ordinary. Rockstar often uses such references, or slight Easter eggs to give an achievement to those who spent time and effort searching.