One of the most common water-type moves you'll see coaches use is Water Pulse. This move may not be the strongest water-type move around, but it has the added benefit of occasionally confusing your opponent. This is where you can find TM03 Water Pulse in Pokémon Brilliant Diamond and Brilliant Pearl.
Before you can find this TM, you must first unlock the Rock Smash and Surf hidden moves. You will get Rock Smash quite early in the game and will be able to use it outside of battle after defeating the Oreburgh City Gym Leader. After progressing in the game for a while, you will automatically get Surf when you reach Celestic Town. To use it outside of battle, you'll need to defeat the Gym Leader in Canalave City.
After getting the hidden moves, head to the Ravaged Path. You should remember this route as the cave connecting Jubilife City and Floaroma Town. Inside the cave, use Rock Smash to access the path on the left. Use Surf on the pool of water and follow it to the end to find the object containing the Water Pulse.