Sonar can get disrupted really easily from any sound wave hanging out in the environment. Those disruptions, also known as "party poopers" by sonar engineers (probably), are

• surface noises.
• other ships.
• sea life.
• self-noise.

All these types of noises make it really hard for sonar to do its job, but they're pretty much unavoidable. At least as long as we're ruling out the idea of nuclear weapons in solving the sounds-caused-by-humanity-and-sea-life issue.

(Even then, though, the self-noise is still going to show up and nuclear waste/the destruction of humanity just isn't worth it.)

Ambient noise is another name for background noise in the environment, typically uniform throughout the space. That means no matter where you go in the local area, the ambient noise is going to be the same.

Surface noise

The things that happen at the surface of the ocean, like ocean waves, can add a lot of background noise to your system. The bigger the waves, the more ambient noise you're going to get. Typically, you'll get noise with a frequency above 300 Hz.

Other Ships

Any ships in the area are going to create some ambient noise that you'll have to hope isn't going to mess with your sonar. In fact, when ships move across your space, you're going to have to deal with the fact that the noise is going to increase a lot—at a low frequency (< 300 Hz). It's also going to be pretty random, so you'll have to expect it even when you least expect it.

Sea Life

Animals like dolphins, whales, and even ones you wouldn't expect (hello snapping shrimp) can create some unpredictable and loud ambient noise. You can't do anything about that noise and you'll only figure out its effects after it's all over.

Good luck.

Self-Made Noise

Sometimes your own ship's electronics can prove to be the Brutus to your system's Caesar. This noise, usually called platform noise, comes from the flow of water across the receiving array of the sonar itself. This self-noise, abbrieved to SN, depends on the frequency and speed of the ship.

Et tu, array?