Mayan Stela #1: Cat Island (734, 694)
Although the next stela on this list is a required part of the story, we have this one listed first. Why? It's found on Cat Island, where you play the mission Sugarcane and Its Yields, so you'll spot it first. When you leave the ship, run up the hill to the south to find it.
Mayan Stela #2: Great Inagua (845, 468)
You'll find this one while playing the story. It's a required part of the This Old Cove mission. You'll also learn how to solve the brief puzzle each one presents, though if you grabbed the one on Cat Island first, you know the puzzles are quite simple.
Mayan Stela #3: Misteriosa (303, 199)
We'll go after this one next, as it's not too far of course when heading to Tulum to meet James Kidd for the mission Nothing Is True - the first time you're asked to cross a particularly large stretch of water. Head southwest to find this island, and be very careful of cliffs while navigating the restricted cove. Once you disembark, you'll find the stela on your way up the massive Maan step pyramid.
Mayan Stela #4: Misteriosa (296, 196)
This is actually found on the same island. Make your way to the western shore to find it.
Mayan Stela #5: Santanillas (217, 245)
This is the last place we'll stop before meeting Kidd. Continue northwest, and be careful in the restricted waters as you approach the island. Look for this beyond the massive rock formation.
Mayan Stela #6: Santanillas (221, 242)
Like some previous pairs of stelae, this one the same island, just to the east. Use the nearby trees and branches to reach it.
Mayan Stela #7: Tulum (70, 405)
When you arrive at Tulum to meet with Kidd, you'll have several stela under your belt. After completing Overrun and Outnumberd, you'll be able to explore the island. It's on the western edge of the ruins.
Mayan Stela #8: Cape Bonavista (179, 593)
For the next stela, we'll loop back north then east on our way to the mission The Forts in Nassau. This route will take you right by Cape Bonavista, where the game began. You'll find it by a waterfall at the northern edge of the map, and yes, it is a bit of a hike.