Use a Potion of Water Breathing
Wear a Turtle Shell for extra underwater breathing time
Enchant a helmet with Respiration
Place Conduits underwater for Conduit Power
Use commands in Creative mode:
`/effect give @p minecraft:water_breathing 99999 0`
`/give @p minecraft:turtle_helmet`
`/give @p minecraft:conduit`
Stay near air pockets or surface for temporary breathing
Use doors, signs, or trapdoors to create air pockets in older versions