For this achievement you need the frost walker enchantment on your boots. Your boots doesn’t have to be diamonds, iron works too. The trickiest part of this achievement is to get the frost walker enchantment. There are two versions: frost walker I and frost walker II, both of them will work. I recommend trading with a Liberian but if you’re lucky there is a chance finding the frost walker book in a desert temple. When your trading with a librarian simply find his workstation and see what he’s offering, if he doesn’t trade the frost walker book then break his workstation. Then place it back and his offering options will be reset. when you get your frost walker book combine your boots and your book on an anvil. This will give you frost walker boots.
The last part is walking on a deep ocean with your boots. A deep ocean is defined by a depth of 30 blocks or more, kelp forest and/or ocean moments.