The Sand Dock is a well known feature located next to the actual Main Biome Dock (from which The Ferry usually takes off from, which was implemented into the game during the January Mega Update). Currently, it is unknown as to what it's purpose actually is. In the future, if the boats will be added the sand dock will be used to leave boats here.
The color of the ramp is primarily brown/tan, and obviously, its texture is Sand, ergo it's title. The ramp stretches down toward the Ocean floor, which is another strange reason why it's predicated upon the theory of Boats. However, near the top of the dock, the dock is flat.
Another speculation that plays along with the Boats Theory is that people will drag their boats up onto land via vehicles, using a hitch, as it seems most likely this will be the purpose. However, this is unknown because there is no Sand Dock featured in the Tropics Biome, which leads players to believe that if Boats were added, this would be the only dock for Boats, the start, and end.
Perhaps in the future, Defaultio will reveal its purpose.