I'M SORRY FOR MY BAD ENGLISH
1. Find stronghold (seed: -27149278)
2. find the library (1098,31,180)
3. find the chests in the library, BUT
do not open them
4. Leave the game and copy the entry in which you found these chests
5. enter the copied world and open the chests, if there is no frost walker book, repeat only the fourth and fifth points. If there is a frost walker book, I congratulate you :) (I managed to do it the second time)
6. if you find the book, create an anvil and shoes, then enchant the shoes using frost walker book
7. find the ocean
(1255, 62, 888) and go on the water with shoes. Achievement should pop out.
( Better location of the deep ocean found Quacky 2 Ducks, thanks bro
santa no Xmas4u's comment:
"If you’re having trouble finding iron, there’s an above ground ravine that intersects a second ravine at 1236, 67, 102 it makes it a lot easier to get the 31 iron if you didn’t get enough from the stronghold chests"