Rhythm and spectrum
Surf combines two things: a broadband wash close to pink noise, which masks, and a slow rhythm well below the pace of breathing at rest. There is no strong trial evidence that the rhythm itself improves focus, but it is the sound people most often describe as slowing them down, and for work that benefits from a lower gear, that is the point.
How it compares
Sea masks speech less well than brown noise, because the level rises and falls with each wave. It masks household noise about as well as rain. If you are in a quiet home office and want something with a pulse, it is a good default; in a café, use brown noise.
Volume
Set it so the loudest wave is just above the room. The troughs between waves will then sit below it, which is what makes the sound feel spacious.