Minecraft wiki entry for Things_not_to_do, associated text: High protection enchanted iron, diamond, or netherite armor
A sword, preferably diamond, netherite or enchanted ( Sharpness for the ender dragon and guardians; Smite for the wither)
A bow, also preferably enchanted (Infinity and Power for the dragon and wither)
An Impaling V trident to kill guardians
Golden apples or health potions
Building blocks or ladders to navigate the terrain
Milk, since it removes the Wither and Mining Fatigue effects (Not needed for the dragon).
Note that Milk also removes beneficial potion effects such as Water Breathing & Night Vision. Be sure to bring extra potions to restore those desired effects that were removed from drinking Milk. This is important to remember when relying on potion effects to explore ocean monuments.
Friends: it is easier with more than one person, so if the player is on a server it is better to fight as a group. On singleplayer, wolves, axolotls, and golems could work as a partial substitute.
A helmet (or Turtle Shell) enchanted with Respiration and Aqua Affinity and boots enchanted with Depth Strider for fighting guardians and maneuvering around ocean monuments
Potion of water breathing (for guardians only)
Potion of Slow Falling for the Ender Dragon fight, to safely drift back down if knocked into the air.
If the player is fighting the wither, it is effective to do so in a low-ceiling cave so the player can have the option of using melee attacks.
Do not fight the wither with melee attacks in the Nether without making sure there is no lava lake below the floor for the fight.