Aqua Affinity

Aqua Affinity is the helmet enchantment that makes mining underwater bearable. With your head submerged and no Aqua Affinity, blocks take five times as long to break, which is why clearing a sand wall or a monument floor turns into a slog long before your air runs out.

It fixes one penalty, not both. Mining while floating — feet off solid ground — is slowed by the same amount again, and Aqua Affinity does nothing about that half of it. Stand on the sea floor, or place a block under yourself before you dig, and an enchanted helmet plus a foothold gives you land speed underwater.

Respiration is the enchantment people confuse it with. Respiration keeps you breathing; Aqua Affinity keeps you digging. They share a helmet without conflict, and serious underwater work wants both, along with a bubble column or a conduit for air.

Practically it pays for itself the first time you drain an ocean monument, dig out a shipwreck or carve a base into the sea floor — anywhere the alternative is placing and re-breaking sponge or sand just to keep your head in air. It comes from an enchanting table as readily as from librarian trades, fishing and loot chests, so it is usually one of the cheaper helmet upgrades to obtain.