Eastshade

For this achievement you will need a coat and the reed boat. Get in the boat of the coast of Blackpaw beach (NW of Tiffmoor Bluffs) and sail east around the northern coast of Eastshade, past the shipwreck. Eventually you will find a beach to disembark on your left (north). Follow the slope up to find the Excavation Site and the achievement will pop.

The previous solution is correct. For those people who prefer a video, enjoy, I made this vídeo to Help, and some tips to help

You’re gonna need it:

Coat - Protects from the Cold at Night - You can buy in Nava for 80 glowstones
Reed Boat - Allows you to navigate in deep water.
You learn after completed the Quest: The Thief of Sinkwood inn
Blackpaw Beach - If you've been there before, there's a Fast travel point