What are the red buoys for?
Those two buoys are indeed the key to a secret area (the big rock formation right near the initial location of the buoys). You will need these to achieve the Cairn Raider achievement.
Cairn Raider - Find the five Cairns on Shipwreck Beach. Lara may find all the needed Cairns after she retrieves the block and tackle for Reyes and Jonah from the first shipwreck.
This link describes the secret and also shows how to get there.
For this section, with two buoys.
Lara needs to manuever two floatation buoys to reach a climb-able wall