All great tips everyone, also remember that not all Biomes will have the Locator Stone. You may have to search a few Plains areas before you find it.

For Yag, the locator stones are usually by stone formations resembling Stonehenge, they can also be found in the stone towers the Goblins build, and I've found one by a single Stonehenge sized stone.

Also, there can be multiple altars for each boss in your seed.

Another tip: When out exploring, before you leave home, build a portal and call it "Home" or "YourNameHome". Then take mats to build another portal when you get to where you're going. That way, if you die, or fill up your inventory, you have an easy way to get back. Any ore you pickup you can leave on your boat for the return trip.

GLHF!