Seabeard's will is found right near the southeastern beach of Whiseman Reef. It is located behind a sign near the beach that reads "Unstable Cliffs". The map of Whiseman Reef is there too. The will looks like a paper in a bottle.
There is another will at the Archeon Temple Ruins. If you look to your left of the ground floor where the tents are, you will see a beach on the left of the island. It will have bones and some grass and it will have the will in a bottle.