So on the Shipwreck Coast map zone I've found two of these buoys and I've no idea what they are for. You can pull them with rope arrows and kick them with F.
I assumed at first that you needed to move them around to allow you to jump up to some ledges, but I don't see anything obvious.
Are they for anything other than environment flavour?
Best Answer
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.
This link describes the secret and also shows how to get there.
For this section, with two buoys.