Minecraft wiki entry for Things_not_to_do, associated text: Enchanted armor (good enchantments include Respiration, Aqua Affinity, Protection, and Depth Strider are essential)
A fairly large amount of food
A sword enchanted with at least Sharpness, but not with Knockback
Conduits, as the large supply of prismarine makes it easy to build a frame. The conduit power grants water breathing, improves visibility, and improves speed.
Iron or better pickaxe (essential, if the player wants to get the blocks of gold in the monument, should be enchanted with Efficiency if the player plans to mine with mining fatigue)
A door‌[Java Edition only] to create an air pocket. The air pocket replenishes the player's air.
Slime blocks, Honey Blocks, or TNT. These blocks can be broken instantly, even with mining fatigue, and can be used to break line of sight with guardians and make them cancel their attacks.