Minecraft wiki entry for Defeating_a_monument, associated text: Using a boat is the easiest way of searching for a monument, especially looking for sea lanterns at night.
A potion of Night Vision helps to see the monument from above the water.
If you have enough, ender pearls can help travel large stretches of water quickly. You can also use a Trident enchanted with Riptide.
Use an Ocean Explorer Map from the cartographer villager.
Use an Elytra in late game.