One of the more common water-type moves you will see trainers use is Water Pulse. This move may not be the strongest water-type move out there, but it comes with the added benefit of occasionally confusing your opponent. Here is where you can find TM03 Water Pulse in Pokémon Brilliant Diamond and Shining Pearl.
Before you can find this TM, you first need to unlock the hidden moves Rock Smash and Surf. You will obtain Rock Smash pretty early in the game and can use it outside of battle after defeating the Oreburgh City gym leader. After progressing through the game further, you will get Surf automatically when you reach Celestic Town. To use it outside of battle, you will need to defeat the gym leader in Canalave City.
After you obtain the hidden moves, head to the Ravaged Path. You should remember this path as the cave that connects Jubilife City and Floaroma Town. Inside the cave, use Rock Smash to gain access to the left path. Use Surf on the pool of water and follow it to the end to find the item containing Water Pulse.