Minecraft wiki entry for Achievement_guide, associated text: If you cannot find deep ocean biomes, buy an ocean explorer map from a cartographer, as monuments will only generate in deep ocean biomes variants. If the deep ocean is deep frozen ocean, the surface will be completely frozen, and you need to break some ice and walk on water, or go inside iceberg, since water only freezes when directly spotted to sunlight, while water in iceberg tunnels doesn't freeze. Once you transform water into frosted ice and walk at least one block, you get the achievement.